Semiconductor device and manufacturing method of the same

ABSTRACT

A semiconductor device includes first and second insulators over a substrate, a semiconductor over the second insulator, first and second conductors over the semiconductor, a third insulator over the semiconductor, a fourth insulator over the third insulator, a third conductor over the fourth insulator, and a fifth insulator over the first insulator, the first conductor and the second conductor. The semiconductor includes first, second, and third regions. The first region overlaps with the third conductor with the third insulator and the fourth insulator positioned therebetween. The second region overlaps with the third conductor with the first conductor, the fourth insulator, and the fifth insulator positioned therebetween. The third region overlaps with the third conductor with the second conductor, the fourth insulator, and the fifth insulator positioned therebetween. The fourth insulator is in contact with a side surface of the fifth insulator in a region overlapping with the semiconductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transistor and a semiconductordevice, and a manufacturing method thereof, for example. The presentinvention relates to a display device, a light-emitting device, alighting device, a power storage device, a memory device, an imagingdevice, a processor, or an electronic device, for example. The presentinvention relates to a method for manufacturing a display device, aliquid crystal display device, a light-emitting device, a memory device,an imaging device, or an electronic device. The present inventionrelates to a driving method of a semiconductor device, a display device,a liquid crystal display device, a light-emitting device, a memorydevice, or an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A display device, a light-emitting device, a lightingdevice, an electro-optical device, a semiconductor circuit, and anelectronic device include a semiconductor device in some cases.

2. Description of the Related Art

In recent years, a transistor including an oxide semiconductor hasattracted attention. It is known that a transistor including an oxidesemiconductor has an extremely low leakage current in an off state. Forexample, a low-power CPU and the like utilizing the characteristics thata leakage current of the transistor including an oxide semiconductor islow is disclosed (see Patent Document 1).

A method for manufacturing a transistor including an oxide semiconductorby forming a gate electrode so as to fill an opening is disclosed (seePatent Document 2 and Patent Document 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2012-257187-   [Patent Document 2] Japanese Published Patent Application No.    2014-241407-   [Patent Document 3] Japanese Published Patent Application No.    2014-240833

SUMMARY OF THE INVENTION

An object is to provide a minute transistor. Another object is toprovide a transistor with low parasitic capacitance. Another object isto provide a transistor with high frequency characteristics. Anotherobject is to provide a transistor with favorable electricalcharacteristics. Another object is to provide a transistor with stableelectrical characteristics. Another object is to provide a transistorwith low off-state current. Another object is to provide a noveltransistor. Another object is to provide a semiconductor deviceincluding the transistor. Another object is to provide a semiconductordevice which can operate at high speed. Another object is to provide anovel semiconductor device. Another object is to provide a moduleincluding the semiconductor device. Another object is to provide anelectronic device including the semiconductor device or the module.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a semiconductor deviceincluding a first insulator over a substrate, a second insulator overthe first insulator, a semiconductor over the second insulator, a firstconductor and a second conductor over the semiconductor, a thirdinsulator over the semiconductor, a fourth insulator over the thirdinsulator, a third conductor over the fourth insulator, and a fifthinsulator over the first insulator, the first conductor, and the secondconductor. The semiconductor includes a first region, a second region,and a third region. The first region includes a region overlapping withthe third conductor with the third insulator and the fourth insulatorpositioned therebetween. The second region includes a region overlappingwith the third conductor with the first conductor, the fourth insulator,and the fifth insulator positioned therebetween. The third regionincludes a region overlapping with the third conductor with the secondconductor, the fourth insulator, and the fifth insulator positionedtherebetween. The fourth insulator includes a region in contact with aside surface of the fifth insulator in a region overlapping with thesemiconductor.

In the above embodiment, the third insulator may have a stacked-layerstructure including two or more layers.

In the above embodiment, the semiconductor may include a CAAC-OS.

In the above embodiment, each of the first to third conductors may havea stacked-layer structure including two or more layers.

In the above embodiment, each of the first conductor and the secondconductor may include regions with different thicknesses.

In the above embodiment, a fourth conductor may be positioned under thefirst insulator.

In the above embodiment, each of the second insulator and the thirdinsulator may contain at least one element contained in thesemiconductor other than oxygen.

In the above embodiment, a top surface of the third insulator may beover top surfaces of the first conductor and the second conductor in across-sectional shape.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of: forming afirst insulator; forming a second insulator over the first insulator;forming a semiconductor over the second insulator; forming a multilayerfilm including the second insulator and the semiconductor by etching apart of the second insulator and a part of the semiconductor; forming afirst conductor over the multilayer film; forming a second conductor anda third conductor by etching a part of the first conductor; forming afourth conductor over the second conductor and the third conductor;forming a third insulator over the fourth conductor and the firstinsulator; dividing the fourth conductor into a fifth conductor and asixth conductor by forming an opening, through which the semiconductorand the first insulator are exposed, in the third insulator and thefourth conductor; forming a fourth insulator over the third insulator,the fifth conductor, the sixth conductor, and the semiconductor; forminga fifth insulator by selectively etching a part of the fourth insulatorso that a region in contact with the semiconductor is left and a part ofa side surface of the third insulator is exposed; forming a sixthinsulator over the fifth insulator and the third insulator; and forminga seventh conductor over the sixth insulator. Each of the secondinsulator and the fourth insulator contain at least one elementcontained in the semiconductor other than oxygen.

In the above embodiment, each of the first and the seventh conductorsmay have a stacked-layer structure including two or more layers.Moreover, an eighth conductor may be further included under the firstinsulator.

In the above embodiment, a top surface of the fifth insulator may beover the top surfaces of the fifth conductor and the sixth conductor ina cross-sectional shape.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of: forming afirst insulator; forming a second insulator over the first insulator;forming a semiconductor over the second insulator; forming a firstconductor over the semiconductor; forming a multilayer film includingthe second insulator, the semiconductor, and the first conductor byetching a part of the second insulator, a part of the semiconductor, anda part of the first conductor; forming a third insulator over themultilayer film; dividing the first conductor into the second conductorand the third conductor by forming an opening, through which thesemiconductor is exposed, in the third insulator and the firstconductor; forming a fourth insulator over the third insulator, thesecond conductor, the third conductor, and the semiconductor; forming afifth insulator by selectively etching a part of the fourth insulator sothat a region in contact with the semiconductor is left and a part of aside surface of the third insulator is exposed; forming a sixthinsulator over the fifth insulator and the third insulator; and forminga fourth conductor over the sixth insulator. Each of the secondinsulator and the fourth insulator contain at least one elementcontained in the semiconductor other than oxygen.

In the above embodiment, each of the first and the fourth conductors mayhave a stacked-layer structure including two or more layers. Moreover, afifth conductor may be further included under the first insulator.

In the above embodiment, a top surface of the fifth insulator may beover the top surfaces of the second conductor and the third conductor ina cross-sectional shape.

In each of the above embodiments, the semiconductor may include aCAAC-OS.

In each of the above embodiments, each of the second conductor and thethird conductor may include regions with different thicknesses.

In each of the above embodiments, the crystallinity of the part of thefourth insulator to be selectively etched is lower than thecrystallinity of the fifth insulator.

In each of the above embodiments, the selective etching is performed bywet etching using an acid solution.

In each of the above embodiments, the fourth insulator may have astacked-layer structure including two or more layers.

A miniaturized transistor can be provided. A transistor with lowparasitic capacitance can be provided. A transistor with high frequencycharacteristics can be provided. A transistor with favorable electricalcharacteristics can be provided. A transistor with stable electricalcharacteristics can be provided. A transistor with low off-state currentcan be provided. A novel transistor can be provided. A semiconductordevice including the transistor can be provided. A semiconductor devicewhich can operate at high speed can be provided. A novel semiconductordevice can be provided. A module including the semiconductor device canbe provided. Furthermore, an electronic device including thesemiconductor device or the module can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views which illustratea transistor of one embodiment of the present invention.

FIGS. 2A to 2C are a top view and cross-sectional views which illustratea transistor of one embodiment of the present invention;

FIGS. 3A and 3B are cross-sectional views each illustrating a part of atransistor of one embodiment of the present invention.

FIGS. 4A and 4B are cross-sectional views each illustrating a part of atransistor of one embodiment of the present invention.

FIGS. 5A to 5C are cross-sectional views each illustrating a part of atransistor of one embodiment of the present invention.

FIGS. 6A to 6C are a top view and cross-sectional views which illustratea transistor of one embodiment of the present invention.

FIG. 7 is a band diagram of one embodiment of the present invention.

FIGS. 8A to 8D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS.

FIGS. 9A to 9D are Cs-corrected high-resolution TEM images of a plane ofa CAAC-OS.

FIGS. 10A to 10C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 11A and 11B show electron diffraction patterns of a CAAC-OS.

FIG. 12 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

FIGS. 13A to 13C are a top view and cross-sectional view illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 14A to 14C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 15A to 15C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 16A to 16C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 17A to 17C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 18A to 18C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 19A to 19C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 20A to 20C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 21A to 21C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 22A to 22D are cross-sectional views illustrating a method formanufacturing a transistor of one embodiment of the present invention.

FIGS. 23A to 23C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 24A to 24C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 25A to 25C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 26A to 26C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 27A to 27C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 28A to 28C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention.

FIGS. 29A and 29B are each a circuit diagram of a memory device of oneembodiment of the present invention.

FIG. 30 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIG. 31 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention.

FIGS. 32A to 32F are circuit diagrams and cross-sectional viewsillustrating a semiconductor device of one embodiment of the presentinvention.

FIG. 33 is a block diagram illustrating a CPU of one embodiment of thepresent invention.

FIG. 34 is a circuit diagram of a memory element of one embodiment ofthe present invention.

FIGS. 35A and 35B are plan views of an imaging device.

FIGS. 36A and 36B are plan views of pixels of an imaging device.

FIGS. 37A and 37B are cross-sectional views of an imaging device.

FIGS. 38A and 38B are cross-sectional views of an imaging device.

FIG. 39 illustrates a configuration example of an RF tag.

FIGS. 40A to 40C are a circuit diagram, a top view, and across-sectional view illustrating a semiconductor device according toone embodiment of the present invention.

FIGS. 41A and 41B are a circuit diagram and a cross-sectional viewillustrating a semiconductor device according to one embodiment of thepresent invention.

FIG. 42 illustrates a display module.

FIGS. 43A and 43B are perspective views illustrating a cross-sectionalstructure of a package using a lead frame interposer.

FIGS. 44A to 44E each illustrate an electronic device of one embodimentof the present invention.

FIGS. 45A to 45D are views each illustrating an electronic device of oneembodiment of the present invention.

FIGS. 46A to 46C are views each illustrating an electronic deviceaccording to one embodiment of the present invention.

FIGS. 47A to 47F illustrate application examples of an RF tag accordingto one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with the reference to the drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that embodiments and detailsdisclosed herein can be modified in various ways. Further, the presentinvention is not construed as being limited to description of theembodiments. In describing structures of the present invention withreference to the drawings, common reference numerals are used for thesame portions in different drawings. Note that the same hatched patternis applied to similar parts, and the similar parts are not especiallydenoted by reference numerals in some cases.

Note that the size, the thickness of films (layers), or the region indrawings is sometimes exaggerated for simplicity.

In this specification, for example, for describing the shape of anobject, the length of one side of a minimal cube where the object fits,or an equivalent circle diameter of a cross section of the object can beinterpreted as the “diameter”, “grain size (diameter)”, “dimension”,“size”, or “width” of the object. The term “equivalent circle diameterof a cross section of the object” refers to the diameter of a perfectcircle having the same area as the cross section of the object.

Note that a voltage refers to a potential difference between a certainpotential and a reference potential (e.g., a ground potential (GND) or asource potential) in many cases. A voltage can be referred to as apotential and vice versa.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. Therefore, for example, the term“first” can be replaced with the term “second”, “third”, or the like asappropriate. In addition, the ordinal numbers in this specification andthe like are not necessarily the same as those which specify oneembodiment of the present invention.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration of lower than 0.1 atomic % isan impurity. When an impurity is contained, the density of states (DOS)may be formed in a semiconductor, the carrier mobility may be decreased,or the crystallinity may be decreased, for example. In the case wherethe semiconductor is an oxide semiconductor, examples of an impuritywhich changes characteristics of the semiconductor include Group 1elements, Group 2 elements, Group 14 elements, Group 15 elements, andtransition metals other than the main components; specifically, thereare hydrogen (included in water), lithium, sodium, silicon, boron,phosphorus, carbon, and nitrogen, for example. In the case of an oxidesemiconductor, oxygen vacancy may be formed by entry of impurities suchas hydrogen. Further, in the case where the semiconductor is a siliconfilm, examples of an impurity which changes characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not limited to one valuein some cases. Therefore, in this specification, the channel length isany one of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, a channel width ofone transistor is not fixed to one value in some cases. Therefore, inthis specification, a channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is increased in some cases. In that case, an effectivechannel width obtained when a channel is actually formed is greater thanan apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

Note that in this specification, the description “A has a shape suchthat an end portion extends beyond an end portion of B” may indicate,for example, the case where at least one of end portions of A ispositioned on an outer side than at least one of end portions of B in atop view or a cross-sectional view. Thus, the description “A has a shapesuch that an end portion extends beyond an end portion of B” can be readas the description “one end portion of A is positioned on an outer sidethan one end portion of B in a top view,” for example.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

In this specification, a term “semiconductor” can be referred to as an“oxide semiconductor”. As the semiconductor, a Group 14 semiconductorsuch as silicon or germanium; a compound semiconductor such as siliconcarbide, germanium silicide, gallium arsenide, indium phosphide, zincselenide, or cadmium sulfide; a carbon nanotube; graphene; or an organicsemiconductor can be used.

Note that in this specification and the like, a “silicon oxynitridefilm” refers to a film that includes oxygen at a higher proportion thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes nitrogen at a higher proportion than oxygen.

Note that in the case where at least one specific example is describedin a diagram or text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the present invention is clear.

Note that in this specification and the like, a content described in atleast a diagram (or may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the present invention is clear.

In addition, contents that are not specified in any text or drawing inthe specification can be excluded from one embodiment of the invention.Alternatively, when the range of a value that is defined by the maximumand minimum values is described, part of the range is appropriatelynarrowed or part of the range is removed, whereby one embodiment of theinvention excluding part of the range can be constituted. In thismanner, it is possible to specify the technical scope of one embodimentof the present invention so that a conventional technology is excluded,for example.

Embodiment 1 Transistor Structure 1

The structures of transistors included in a semiconductor device ofembodiments of the present invention will be described below.

FIGS. 1A to 1C are a top view and cross-sectional views of thesemiconductor device of one embodiment of the present invention. FIG. 1Ais the top view. FIG. 1B is a cross-sectional view taken alongdashed-dotted line A1-A2 in FIG. 1A, which illustrates a cross-sectionalshape in the channel length direction. FIG. 1C is a cross-sectional viewtaken along dashed-dotted line A3-A4 in FIG. 1A, which illustrates across-sectional shape in the channel width direction. Note that forsimplification of the drawing, some components in the top view in FIG.1A are not illustrated. The paths through which the excess oxygen(referred to as exO) passes are denoted by arrows in FIGS. 1B and 1C.

The transistor illustrated in FIGS. 1A to 1C includes a conductor 413over a substrate 400, an insulator 402 over the conductor 413, aninsulator 406 a over the insulator 402, a semiconductor 406 b over theinsulator 406 a, a conductor 416 a 1 and a conductor 416 a 2 eachincluding a region in contact with a top surface of the semiconductor406 b, a conductor 416 b 1 over the conductor 416 a 1, a conductor 416 b2 over the conductor 416 a 2, an insulator 410 in contact with topsurfaces of the conductors 416 b 1 and 416 b 2, an insulator 406 c incontact with a top surface and a side surface of the semiconductor 406b, an insulator 406 d in contact with a top surface of the insulator 406c, an insulator 412 in contact with the insulator 406 d and theinsulator 410, a conductor 404 over the semiconductor 406 b with theinsulator 412, the insulator 406 c, and the insulator 406 d positionedtherebetween, and an insulator 408 over the insulator 412 and theconductor 404. Note that the insulator 406 c may be in contact with theinsulator 402.

A mixed region 414 containing excess oxygen is formed in the vicinity ofthe interface between the insulator 408 and the insulator 410 in somecases.

The insulator 406 c and the insulator 406 d preferably contain at leastone element contained in the semiconductor 406 b other than oxygen. Thiscan reduce generation of defects at each of the interface between thesemiconductor 406 b and the insulator 406 c and that between theinsulator 406 c and the insulator 406 d. Furthermore, the crystallinityof the insulators 406 c and 406 d can be improved.

It is preferable that the semiconductor 406 b and the insulator 406 ceach include a CAAC-OS which will be described later. In addition, theinsulator 406 d preferably includes a CAAC-OS. Furthermore, theinsulator 406 a preferably includes a CAAC-OS.

In the transistor, the conductor 404 serves as a first gate electrode.The conductor 404 can have a stacked-layer structure including aconductor that is less likely to transmit oxygen. For example, forming aconductor that is less likely to transmit oxygen in a lower layer canprevent decrease in conductivity due to oxidation of the conductor 404.In addition, the insulator 412 serves as a first gate insulator.

The conductor 413 serves as a second gate electrode. The conductor 413can have a stacked-layer structure including a conductive film that isless likely to transmit oxygen. The stacked-layer structure including aconductive film that is less likely to transmit oxygen can preventdecrease in conductivity due to oxidation of the conductor 413. Theinsulator 402 serves as a second gate insulator. The potential appliedto the conductor 413 can control the threshold voltage of thetransistor. When the first gate electrode is electrically connected tothe second gate electrode, the current in a conducting state (on-statecurrent) can be increased. Note that the function of the first gateelectrode and that of the second gate electrode may be interchanged.

FIGS. 2A to 2C illustrate an example in which the first gate electrodeis electrically connected to the second gate electrode. As illustratedin FIG. 2C, by forming an opening in the insulator 402, the insulator410, and the insulator 412, the conductor 413 and the conductor 404 areelectrically connected to each other.

The conductors 416 a 1 and 416 a 2 serve as a source electrode and adrain electrode. It is preferable that each of the conductors 416 a 1and 416 a 2 preferably has regions with different thicknesses. Forexample, the thickness of a region in contact with the insulator 406 c,the insulator 406 d, or the insulator 412 of the conductor 416 a 1 andthe conductor 416 a 2 is preferably small. In addition, the conductors416 a 1 and 416 a 2 can each have a stacked-layer structure including aconductor that is less likely to transmit oxygen. For example, forming aconductor that is less likely to transmit oxygen in an upper layer canprevent decrease in conductivity due to oxidation of the conductor 416 a1 and the conductor 416 a 2. Note that conductivity of the conductor canbe measured by a two-terminal method or the like.

Therefore, the resistance of the semiconductor 406 b can be controlledby a potential applied to the conductor 404. That is, conduction ornon-conduction between the conductors 416 a 1 and 416 a 2 can becontrolled by the potential applied to the conductor 404.

As illustrated in FIG. 1B, the top surface of the semiconductor 406 b isin contact with the conductors 416 a 1 and 416 a 2. In addition, thesemiconductor 406 b can be electrically surrounded by an electric fieldof the conductor 404 serving as the gate electrode. A structure in whicha semiconductor is electrically surrounded by an electric field of agate electrode is referred to as a surrounded channel (s-channel)structure. Therefore, a channel is formed in the entire semiconductor406 b in some cases. In the s-channel structure, a large amount ofcurrent can flow between a source and a drain of the transistor, so thatan on-state current can be increased. In addition, since thesemiconductor 406 b is surrounded by the electric field of the conductor404, an off-state current can be decreased.

The transistor in this embodiment can also be referred to as atrench-gate self-aligned s-channel FET (TGSA s-channel FET) because theregion serving as a gate electrode is formed in a self-aligned manner tofill the opening formed in the insulator 410 and the like.

In the transistor in this embodiment, the insulator 410 is provided in aregion where the conductors 416 a 1 and 416 b 1 and the conductor 404overlap with each other. Accordingly, the length between the conductors416 a 1 and 416 b 1 and the conductor 404 becomes longer, whereby theparasitic capacitance generated between the conductors 416 a 1 and 416 b1 and the conductor 404 can be reduced.

FIGS. 3A and 3B are each an enlarged view of a portion of a transistor.FIG. 3A illustrates an enlarged portion of the transistor of oneembodiment of the present invention. FIG. 3B illustrates an enlargedportion of a comparative transistor.

In FIG. 3A, the length of the region of the bottom surface of theconductor 404 serving as a gate electrode facing and parallel to the topsurface of the semiconductor 406 b with the insulator 412, the insulator406 c, and the insulator 406 d positioned therebetween is denoted as agate line width 404 w 1. The length of the region where thesemiconductor 406 b, the conductor 416 a 1, the conductor 416 b 1, theinsulator 410, the insulator 412, and the conductor 404 overlap with oneanother is denoted as 416 w 1. The length of the region where thesemiconductor 406 b, the conductor 416 a 2, the conductor 416 b 2, theinsulator 410, the insulator 412, and the conductor 404 overlap with oneanother is denoted as 416 w 2. In cross-sectional views in FIGS. 3A and3B, the shortest distance between the region of 416 w 1 and the regionof 404 w 1 is 404 w 2. The shortest distance between the region of 416 w2 and the region of 404 w 1 is 404 w 3. The sum of the lengths 404 w 1,404 w 2, and 404 w 3 is denoted as 403 w. In FIG. 3A, the length of theconductor 404 is the sum of 416 w 1, 404 w 2, 404 w 1, 404 w 3, and 416w 2.

In FIG. 3B, the length of the region of the bottom surface of theconductor 404 serving as a gate electrode facing and parallel to the topsurface of the semiconductor 406 b with the insulator 412, the insulator406 c, and the insulator 406 d positioned therebetween is denoted as agate line width 405 w 1. The length of the region where thesemiconductor 406 b, the conductor 416 a 1, the conductor 416 b 1, theinsulator 410, the insulator 412, and the conductor 404 overlap with oneanother is denoted as 416 w 3. The length of the region where thesemiconductor 406 b, the conductor 416 a 2, the conductor 416 b 4, theinsulator 410, the insulator 412, and the conductor 404 overlap with oneanother is denoted as 416 w 4. The shortest distance between the regionof 416 w 3 and the region of 405 w 1 is denoted as 405 w 2. The shortestdistance between the region of 416 w 4 and the region of 405 w 1 isdenoted as 405 w 3. The sum of the lengths 405 w 1, 405 w 2, and 405 w 3is denoted as 405 w. In FIG. 3B, the length of the conductor 404 is thesum of 416 w 3, 405 w 2, 405 w 1, 405 w 3, and 416 w 4.

In the transistors in FIGS. 3A and 3B, the conductor 404 serving as agate electrode partly overlap with the conductors 416 a 1 and 416 a 2serving as a source electrode and a drain electrode. However, in thesemiconductor 406 b, the region of 404 w 2 and the region of 404 w 3 inFIG. 3A and the region of 405 w 2 and the region of 405 w 3 in FIG. 3Bare regions where an electric field from the gate electrode is smallbecause the distance between the conductor 404 and the semiconductor 406b is long. The regions of the semiconductor 406 b have low conductivityin a channel region formed between the conductor 416 a 1 and theconductor 416 a 2. The on-state current of the transistor becomes smalldue to the regions; thus, the regions are preferably small.

In the transistor of this embodiment illustrated in FIG. 3A, theinsulator 406 c and the insulator 406 d are not formed to cover a sidesurface of the opening but in contact with a part of the side surface.In the transistor of this embodiment illustrated in FIG. 3A, theinsulator 406 c and the insulator 406 d are not formed on a side surfaceof an opening formed in the insulator 410. Thus, the transistor in FIG.3A is preferable because the transistor has smaller region with lowconductivity than the comparative transistor in FIG. 3B.

In a cross-sectional view in FIG. 3A, the top surface of the insulator406 d is preferably over the top surfaces of the conductors 416 a 1 and416 a 2, more preferably over the top surfaces of the conductors 416 b 1and the conductor 416 b 2. In addition, in a cross-sectional shape, thetop surface of the insulator 406 c is preferably over the top surfacesof the conductors 416 a 1 and 416 a 2, more preferably over the topsurfaces of the conductors 416 b 1 and the conductor 416 b 2. Thus, evenin the case where part of the semiconductor 406 b is recessed asillustrated by the dotted circles in the enlarged view of part of thetransistor illustrated in FIG. 4A, the recessed part can be filled byforming the insulator 406 c. Alternatively, even in the case where partsof the conductors 416 a 1 and 416 a 2 are recessed as illustrated by thedotted circles in the enlarged view of part of the transistorillustrated in FIG. 4B, the recessed parts can be filled with theinsulator 406 c. Alternatively, even in the case where parts of thesemiconductor 406 b, the conductor 416 a 1, and the conductor 416 a 2are recessed as illustrated in the dotted circles in the enlarged viewof part of the transistor illustrated in FIG. 5A, the recessed parts canbe filled by forming the insulator 406 c.

Alternatively, a structure without the conductor 413 may be used (seeFIG. 5B). Alternatively, as illustrated in FIG. 5C, the insulator 412may protrude from the conductor 404.

The transistor of this embodiment has, although not limited to, astructure in which the insulators 406 c and 406 d are provided over thesemiconductor 406 b. For example, only an insulator 406 e may beprovided over the semiconductor 406 b (see FIGS. 6A to 6C).

As the substrate 400, an insulator substrate, a semiconductor substrate,or a conductor substrate may be used, for example. As the insulatorsubstrate, a glass substrate, a quartz substrate, a sapphire substrate,a stabilized zirconia substrate (e.g., an yttria-stabilized zirconiasubstrate), or a resin substrate is used, for example. As thesemiconductor substrate, a single material semiconductor substrate ofsilicon, germanium, or the like or a compound semiconductor substrate ofsilicon carbide, silicon germanium, gallium arsenide, indium phosphide,zinc oxide, gallium oxide, or the like is used, for example. Asemiconductor substrate in which an insulator region is provided in theabove semiconductor substrate, e.g., a silicon on insulator (SOI)substrate or the like is used. As the conductor substrate, a graphitesubstrate, a metal substrate, an alloy substrate, a conductive resinsubstrate, or the like is used. A substrate including a metal nitride, asubstrate including a metal oxide, or the like is used. An insulatorsubstrate provided with a conductor or a semiconductor, a semiconductorsubstrate provided with a conductor or an insulator, a conductorsubstrate provided with a semiconductor or an insulator, or the like isused. Alternatively, any of these substrates over which an element isprovided may be used. As the element provided over the substrate, acapacitor, a resistor, a switching element, a light-emitting element, amemory element, or the like is used.

Alternatively, a flexible substrate may be used as the substrate 400. Asa method for providing a transistor over a flexible substrate, there isa method in which the transistor is formed over a non-flexible substrateand then the transistor is separated and transferred to the substrate400 which is a flexible substrate. In that case, a separation layer ispreferably provided between the non-flexible substrate and thetransistor. As the substrate 400, a sheet, a film, or a foil containinga fiber may be used. The substrate 400 may have elasticity. Thesubstrate 400 may have a property of returning to its original shapewhen bending or pulling is stopped. Alternatively, the substrate 400 mayhave a property of not returning to its original shape. The substrate400 has a region with a thickness of, for example, greater than or equalto 5 μm and less than or equal to 700 μm, preferably greater than orequal to 10 μm and less than or equal to 500 μm, more preferably greaterthan or equal to 15 μm and less than or equal to 300 μm. When thesubstrate 400 has a small thickness, the weight of the semiconductordevice including the transistor can be reduced. When the substrate 400has a small thickness, even in the case of using glass or the like, thesubstrate 400 may have elasticity or a property of returning to itsoriginal shape when bending or pulling is stopped. Therefore, an impactapplied to the semiconductor device over the substrate 400, which iscaused by dropping or the like, can be reduced. That is, a durablesemiconductor device can be provided.

For the substrate 400 which is a flexible substrate, metal, an alloy,resin, glass, or fiber thereof can be used, for example. The flexiblesubstrate 400 preferably has a lower coefficient of linear expansionbecause deformation due to an environment is suppressed. The flexiblesubstrate 400 is formed using, for example, a material whose coefficientof linear expansion is lower than or equal to 1×10⁻³/K, lower than orequal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, and acrylic. In particular, aramid ispreferably used for the flexible substrate 400 because of its lowcoefficient of linear expansion.

Note that electrical characteristics of the transistor can be stabilizedwhen the transistor is surrounded by an insulator with a function ofblocking oxygen and impurities such as hydrogen. For example, aninsulator with a function of blocking oxygen and impurities such ashydrogen may be used as the insulator 408.

An insulator with a function of blocking oxygen and impurities such ashydrogen may have a single-layer structure or a stacked-layer structureincluding an insulator containing, for example, boron, carbon, nitrogen,oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine,argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium,hafnium, or tantalum may be used.

For example, the insulator 408 may be formed of aluminum oxide,magnesium oxide, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide. Note that theinsulator 408 preferably contains aluminum oxide. For example, when theinsulator 408 is formed using plasma containing oxygen, oxygen can beadded to the insulator 410 to be a base layer of the insulator 408 or aside surface of the insulator 412. The added oxygen becomes excessoxygen in the insulator 410 or the insulator 412. When the insulator 408contains aluminum oxide, entry of impurities such as hydrogen into thesemiconductor 406 b can be inhibited. In addition, when the insulator408 contains aluminum oxide, outward diffusion of excess oxygen that isadded to the insulator 410 and the insulator 412 can be reduced, forexample.

The insulator 402 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including an insulator containingboron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. For example, the insulator402 preferably contains silicon oxide or silicon oxynitride.

Note that the insulator 410 preferably includes an insulator with lowrelative dielectric constant. For example, the insulator 410 preferablycontains silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, silicon oxide to which fluorine is added, silicon oxideto which carbon is added, silicon oxide to which carbon and nitrogen areadded, silicon oxide having pores, a resin, or the like. Alternatively,the insulator 410 preferably has a stacked-layer structure of a resinand one of the following materials: silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, silicon oxide to which fluorineis added, silicon oxide to which carbon is added, silicon oxide to whichcarbon and nitrogen are added, and silicon oxide having pores. Whensilicon oxide or silicon oxynitride, which is thermally stable, iscombined with a resin, the stacked-layer structure can have thermalstability and low relative dielectric constant. Examples of the resininclude polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, and acrylic.

The insulator 412 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including an insulator containingboron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. For example, for theinsulator 412, a material containing silicon oxide or silicon oxynitrideis preferably used.

Note that the insulator 412 preferably contains an insulator with a highrelative dielectric constant. For example, the insulator 412 preferablycontains gallium oxide, hafnium oxide, oxide containing aluminum andhafnium, oxynitride containing aluminum and hafnium, oxide containingsilicon and hafnium, oxynitride containing silicon and hafnium, or thelike. The insulator 412 preferably has a stacked-layer structurecontaining silicon oxide or silicon oxynitride and an insulator with ahigh relative dielectric constant. Since silicon oxide and siliconoxynitride have thermal stability, combination of silicon oxide orsilicon oxynitride with an insulator with a high relative dielectricconstant allows the stacked-layer structure to be thermally stable andhave a high relative dielectric constant. For example, when an aluminumoxide, a gallium oxide, or a hafnium oxide of the insulator 412 is onthe insulators 406 c and 406 d side, entry of silicon included in thesilicon oxide or the silicon oxynitride into the semiconductor 406 b canbe suppressed. When silicon oxide or silicon oxynitride is on theinsulators 406 c and 406 d side, for example, trap centers might beformed at the interface between aluminum oxide, gallium oxide, orhafnium oxide and silicon oxide or silicon oxynitride. The trap centerscan shift the threshold voltage of the transistor in the positivedirection by trapping electrons in some cases.

Each of the conductors 416 a 1 and 416 a 2 may be formed to have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, platinum, silver, indium, tin, tantalum, andtungsten. For example, an alloy film or a compound film may be used: aconductor containing aluminum, a conductor containing copper andtitanium, a conductor containing copper and manganese, a conductorcontaining indium, tin, and oxygen, a conductor containing titanium andnitrogen, or the like may be used. Furthermore, the conductors 416 b 1and 416 b 2 may be formed in a similar manner to that of the conductors416 a 1 and 416 a 2.

Each of the conductors 404 and 413 may be formed to have a single-layerstructure or a stacked-layer structure including a conductor containing,for example, one or more kinds of boron, nitrogen, oxygen, fluorine,silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt,nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum,ruthenium, silver, indium, tin, tantalum, and tungsten. For example, analloy film or a compound film may be used: a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

An oxide semiconductor is preferably used as the semiconductor 406 b.However, silicon (including strained silicon), germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, an organic semiconductor, or the likecan be used in some cases.

As the insulator 406 a, the insulator 406 c, and the insulator 406 d,oxides containing one or more elements other than oxygen contained inthe semiconductor 406 b are preferably used. However, silicon (includingstrained silicon), germanium, silicon germanium, silicon carbide,gallium arsenide, aluminum gallium arsenide, indium phosphide, galliumnitride, an organic semiconductor, or the like can be used in somecases.

The semiconductor 406 b is an oxide semiconductor containing indium, forexample. The semiconductor 406 b can have high carrier mobility(electron mobility) by containing indium, for example. The semiconductor406 b preferably contains an element M. The element M is preferablyaluminum, gallium, yttrium, tin, or the like. Other elements which canbe used as the element M are boron, silicon, titanium, iron, nickel,germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, magnesium, tungsten, and the like. Note that two or more ofthe above elements may be used in combination as the element M. Theelement M is an element having high bonding energy with oxygen, forexample. The element M is an element whose bonding energy with oxygen ishigher than that of indium. The element M is an element that canincrease the energy gap of the oxide semiconductor, for example.Furthermore, the semiconductor 406 b preferably contains zinc. When theoxide semiconductor contains zinc, the oxide semiconductor is easily tobe crystallized, for example.

Note that the semiconductor 406 b is not limited to the oxidesemiconductor containing indium. The semiconductor 406 b may be, forexample, an oxide semiconductor which does not contain indium andcontains zinc, an oxide semiconductor which does not contain indium andcontains gallium, or an oxide semiconductor which does not containindium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide.

For the semiconductor 406 b, an oxide with a wide energy gap may beused. For example, the energy gap of the semiconductor 406 b is greaterthan or equal to 2.5 eV and less than or equal to 4.2 eV, preferablygreater than or equal to 2.8 eV and less than or equal to 3.8 eV, morepreferably greater than or equal to 3 eV and less than or equal to 3.5eV.

For example, the insulator 406 a, the insulator 406 c, the insulator 406d, and the insulator 406 e are oxides including one or more elements, ortwo or more elements other than oxygen included in the semiconductor 406b. Since the insulator 406 a, the insulator 406 c, and the insulator 406d each include one or more elements, or two or more elements other thanoxygen included in the semiconductor 406 b, a defect state is lesslikely to be formed at the interface between the insulator 406 a and thesemiconductor 406 b and the interface between the semiconductor 406 band the insulator 406 c.

As the semiconductor 406 b, an oxide having an electron affinity higherthan those of the insulators 406 a, 406 c, and 406 d is used. Forexample, as the semiconductor 406 b, an oxide having an electronaffinity higher than those of the insulators 406 a, 406 c, and 406 d by0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and0.7 eV or lower, or further preferably 0.15 eV or higher and 0.4 eV orlower is used. Note that the electron affinity refers to an energy gapbetween the vacuum level and the bottom of the conduction band.Furthermore, the insulator 406 c preferably has higher electron affinitythan the insulator 406 d.

When gate voltage is applied to such a transistor in which the insulator406 a is placed under the semiconductor 406 b and the insulators 406 cand 406 d are placed over the semiconductor 406 b, a channel is formedin the semiconductor 406 b whose electron affinity is the highest amongthe insulator 406 a, the semiconductor 406 b, the insulator 406 c, andthe insulator 406 d. In this manner, a buried channel structure isformed.

In some cases, there is a mixed region of the insulator 406 a and thesemiconductor 406 b between the insulator 406 a and the semiconductor406 b. Furthermore, in some cases, there is a mixed region of thesemiconductor 406 b and the insulator 406 c between the semiconductor406 b and the insulator 406 c. Furthermore, in some cases, there is amixed region of the semiconductor 406 b and the insulator 406 c betweenthe semiconductor 406 b and the insulator 406 c. The mixed region has alow density of defect states. For that reason, in a band diagram of astack including the insulator 406 a, the semiconductor 406 b, theinsulator 406 c, and the insulator 406 d (see FIG. 7), energy changescontinuously at each interface and in the vicinity of the interface(continuous junction). Note that boundaries of the insulator 406 a, thesemiconductor 406 b, the insulator 406 c, and the insulator 406 d arenot clear in some cases.

At this time, electrons move mainly in the semiconductor 406 b, not inthe insulators 406 a, 406 c, and 406 d.

As factors of inhibiting electron movement are decreased, the on-statecurrent of the transistor can be increased. Electron movement isinhibited, for example, in the case where physical unevenness in achannel formation region is large.

To increase the on-state current of the transistor, for example, rootmean square (RMS) roughness with a measurement area of 1 μm×1 μm of thetop surface or the bottom surface of the semiconductor 406 b (aformation surface; here, the top surface of the insulator 406 a) is lessthan 1 nm, preferably less than 0.6 nm, further preferably less than 0.5nm, still further preferably less than 0.4 nm. The average surfaceroughness (also referred to as Ra) with the measurement area of 1 μm×1μm is less than 1 nm, preferably less than 0.6 nm, further preferablyless than 0.5 nm, still further preferably less than 0.4 nm. The maximumdifference (P-V) with the measurement area of 1 μm×1 μm is less than 10nm, preferably less than 9 nm, further preferably less than 8 nm, stillfurther preferably less than 7 nm. RMS roughness, Ra, and P-V can bemeasured using a scanning probe microscope SPA-500 manufactured by SIINano Technology Inc.

The above four-layer structure is an example. For example, astacked-layer structure in which one or more of the insulators describedas examples of the insulator 406 a, the insulator 406 c, and theinsulator 406 d is provided under or over the insulator 406 a, under orover the insulator 406 c, or under or over the insulator 406 d may beemployed.

<Structure of Oxide Semiconductor>

Structures of an oxide semiconductor that can be used as the abovesemiconductor will be described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a CAAC-OS, apolycrystalline oxide semiconductor, an nc-OS, an amorphous-like oxidesemiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS will be described.

The CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur. The pellet is largerthan or equal to 1 nm, preferably larger than or equal to 3 nm, andfurther preferably larger than or equal to 6 nm.

A CAAC-OS observed with TEM will be described below. FIG. 8A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 8B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 8A. FIG. 8B shows that metal atoms are arranged in alayered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which a CAAC-OS film is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film.

As shown in FIG. 8B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 8C. FIGS. 8B and 8C prove that the size of apellet is greater than or equal to 1 nm or greater than or equal to 3nm, and the size of a space caused by tilt of the pellets isapproximately 0.8 nm. Therefore, the pellet can also be referred to as ananocrystal (nc). Furthermore, the CAAC-OS can also be referred to as anoxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 8D). The part in which the pellets are tilted as observed inFIG. 8C corresponds to a region 5161 shown in FIG. 8D.

FIG. 9A shows a Cs-corrected high-resolution TEM image of a plane of theCAAC-OS observed from a direction substantially perpendicular to thesample surface.

FIGS. 9B, 9C, and 9D are enlarged Cs-corrected high-resolution TEMimages of regions (1), (2), and (3) in FIG. 9A, respectively. FIGS. 9B,9C, and 9D indicate that metal atoms are arranged in a triangular,quadrangular, or hexagonal configuration in a pellet. However, there isno regularity of arrangement of metal atoms between different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) will be described.

For example, when the structure of a CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method, a peak appears at adiffraction angle (28) of around 31° as shown in FIG. 10A. This peak isderived from the (009) plane of the InGaZnO₄ crystal, which indicatesthat crystals in the CAAC-OS have c-axis alignment, and that the c-axesare aligned in a direction substantially perpendicular to the formationsurface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 28 is around 36°, in addition tothe peak at 28 of around 31°. The peak at 28 of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 28 is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray beam is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 28 isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 28 fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (4) axis), as shown in FIG. 10B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when 4) scan is performed with2θ fixed at around 56°, as shown in FIG. 10C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction will be described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 11A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 11B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 11B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 11B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 11B is considered to be derived from the (110)plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. For example,impurities contained in the oxide semiconductor might serve as carriertraps or carrier generation sources. Furthermore, oxygen vacancies inthe oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with a low carrier density. Specifically, anoxide semiconductor with a carrier density of lower than 8×10¹¹/cm³,preferably lower than 1×10¹¹/cm³, further preferably lower than1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³ can be used. Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary cannot be clearly observedin some cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakindicating a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of spots is shownin a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-like OS>

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 12 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 12 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 12, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 12, the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS might beinduced by electron irradiation, for example. In contrast, in the nc-OSand the CAAC-OS, it is shown that growth of the crystal part is hardlyinduced by electron irradiation. Therefore, the a-like OS has anunstable structure as compared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS, for example.

<Method 1 for Manufacturing Transistor>

A method for manufacturing the transistor of the present inventionillustrated in FIGS. 1A to 1C is described below with reference to FIGS.13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS.17A to 17C, FIGS. 18A to 18C, FIGS. 19A to 19C, FIGS. 20A to 20C, FIGS.21A to 21C, and FIGS. 22A to 22D.

First, the substrate 400 is prepared.

Next, a conductor is formed over the substrate 400 and is then processedby a photolithography method or the like to form the conductor 413. Theconductor to be the conductor 413 can be formed by a sputtering method,a CVD method, an MBE method, a PLD method, an ALD method, or the like.The conductor 413 may have a multilayer structure including a conductorthat is less likely to transmit oxygen. Another method for forming theconductor 413 is described. An insulator to be the insulator 402 isformed over the substrate 400. An opening is formed in the insulator402, whereby a conductor to be the conductor 413 is formed over theinsulator 402. The conductor 413 may be embedded to the opening in theinsulator 402 by chemical mechanical polishing (CMP) or the like.

In the photolithography method, first, a resist is exposed to lightthrough a photomask. Next, a region exposed to light is removed or leftusing a developing solution, so that a resist mask is formed. Then,etching through the resist mask is conducted. As a result, a conductor,a semiconductor, an insulator, or the like can be processed into adesired shape. The resist mask is formed by, for example, exposure ofthe resist to light using KrF excimer laser light, ArF excimer laserlight, extreme ultraviolet (EUV) light, or the like. Alternatively, aliquid immersion technique in which a portion between a substrate and aprojection lens is filled with liquid (e.g., water) to perform lightexposure may be employed. An electron beam or an ion beam may be usedinstead of the above-mentioned light. Note that a photomask is notnecessary in the case of using an electron beam or an ion beam. For theremoval of the resist mask, dry etching treatment such as ashing, wetetching treatment, or both can be used.

As a dry etching apparatus, a capacitively coupled plasma (CCP) etchingapparatus including parallel plate type electrodes can be used. Thecapacitively coupled plasma etching apparatus including the parallelplate type electrodes may have a structure in which a high-frequencypower is applied to one of the parallel plate type electrodes.Alternatively, the capacitively coupled plasma etching apparatus mayhave a structure in which different high-frequency power is applied toone of the parallel plate type electrodes. Alternatively, thecapacitively coupled plasma etching apparatus may have a structure inwhich high-frequency power with the same frequency is applied to theparallel plate type electrodes. Alternatively, the capacitively coupledplasma etching apparatus may have a structure in which high-frequencypower with different frequencies is applied to the parallel plate typeelectrodes. Alternatively, a dry etching apparatus including ahigh-density plasma source can be used. As the dry etching apparatusincluding a high-density plasma source, an inductively coupled plasma(ICP) etching apparatus can be used, for example

Next, the insulator 402 is formed. The insulator 402 may be formed by asputtering method, a chemical vapor deposition (CVD) method, a molecularbeam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, anatomic layer deposition (ALD) method, or the like.

CVD methods can be classified into a plasma enhanced CVD (PECVD) methodusing plasma, a thermal CVD (TCVD) method using heat, a photo CVD methodusing light, and the like. Moreover, the CVD method can include a metalCVD (MCVD) method and a metal organic CVD (MOCVD) method depending on asource gas.

By using the PECVD method, a high-quality film can be formed at arelatively low temperature. Furthermore, a thermal CVD method does notuse plasma and thus causes less plasma damage to an object to beprocessed. For example, a wiring, an electrode, an element (e.g.,transistor or capacitor), or the like included in a semiconductor devicemight be charged up by receiving charges from plasma. In that case,accumulated charges might break the wiring, electrode, element, or thelike included in the semiconductor device. By contrast, when a thermalCVD method not using plasma is employed, such damage due to exposure toplasma is not caused and the yield of the semiconductor device can beincreased. In a thermal CVD method, an object to be processed is notexposed to plasma during deposition, so that a film with few defects canbe obtained.

An ALD method also causes less plasma damage to an object to beprocessed. An ALD method does not cause plasma damage during deposition,so that a film with few defects can be obtained.

Unlike in a deposition method in which particles ejected from a targetor the like are deposited, in a CVD method and an ALD method, a film isformed by reaction at a surface of an object to be processed. Thus, aCVD method and an ALD method enable favorable step coverage almostregardless of the shape of an object to be processed. In particular, anALD method enables excellent step coverage and excellent thicknessuniformity and can be favorably used for covering a surface of anopening with a high aspect ratio, for example. On the other hand, an ALDmethod has a low deposition rate; thus, it is sometimes preferable tocombine an ALD method with another deposition method with a highdeposition rate such as a CVD method.

When a CVD method or an ALD method is used, composition of a film to beformed can be controlled with a flow rate ratio of the source gases. Forexample, by a CVD method or an ALD method, a film with a certaincomposition can be formed depending on a flow rate ratio of the sourcegases. Moreover, with a CVD method or an ALD method, by changing theflow rate ratio of the source gases while forming the film, a film whosecomposition is continuously changed can be formed. In the case where thefilm is formed while changing the flow rate ratio of the source gases,as compared to the case where the film is formed using a plurality ofdeposition chambers, time taken for the film formation can be reducedbecause time taken for transfer and pressure adjustment is omitted.Thus, semiconductor devices can be manufactured with improvedproductivity.

Next, treatment to add oxygen to the insulator 402 may be performed. Forthe treatment to add oxygen, an ion implantation method, a plasmatreatment method, or the like can be used. Note that oxygen added to theinsulator 402 is excess oxygen.

Next, an insulator to be the insulator 406 a is formed over theinsulator 402. The insulator to be the insulator 406 a can be formed bya sputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. It is particularly preferable to use afacing-target sputtering apparatus. Note that in this specification andthe like, deposition using a facing-target sputtering apparatus can alsobe referred to as vapor deposition sputtering (VDSP).

The use of the facing-target sputtering apparatus can reduce plasmadamage induced during deposition of the insulator. Thus, oxygenvacancies in the insulator can be reduced. In addition, the use of thefacing-target sputtering apparatus allows deposition in high vacuum. Inthat case, impurity concentration (e.g., concentration of hydrogen, arare gas (such as argon), or water) in the deposited insulator can bereduced.

Alternatively, a sputtering apparatus including an inductively-coupledantenna conductor plate may be used. Thus, a large film with highuniformity can be formed with a high deposition rate.

Deposition is preferably performed using a gas containing oxygen, a raregas, a gas containing nitrogen, or the like. As the gas containingnitrogen, nitrogen (N₂), dinitrogen oxide (N₂O), ammonia (NH₃), or thelike may be used, for example.

Next, treatment to add oxygen to the insulator to be the insulator 406 amay be performed. An ion implantation method, a plasma treatment method,or the like can be used for the treatment to add oxygen. Note thatoxygen added to the insulator to be the insulator 406 a is excessoxygen.

Next, the semiconductor to be the semiconductor 406 b is formed over theinsulator to be the insulator 406 a. The semiconductor can be formed bya sputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. It is particularly preferable to use afacing-target sputtering apparatus.

The use of the facing-target sputtering apparatus can reduce plasmadamage induced during deposition of the semiconductor. Accordingly,oxygen vacancies in the semiconductor can be reduced. In addition, theuse of the facing-target sputtering apparatus allows deposition in highvacuum. In that case, impurity concentration (e.g., concentration ofhydrogen, a rare gas (such as argon), or water) in the depositedsemiconductor can be reduced.

Alternatively, a sputtering apparatus including an inductively-coupledantenna conductor plate may be used. Thus, a large film with highuniformity can be formed with a high deposition rate.

Deposition is preferably performed using a gas containing oxygen, a raregas, a gas containing nitrogen, or the like. As the gas containingnitrogen, nitrogen (N₂), dinitrogen oxide (N₂O), or ammonia (NH₃) may beused, for example.

Next, first heat treatment is preferably performed. The first heattreatment can be performed at a temperature higher than or equal to 250°C. and lower than or equal to 650° C., preferably higher than or equalto 450° C. and lower than or equal to 600° C. The first heat treatmentis performed in an inert gas atmosphere or an atmosphere containing anoxidizing gas at 10 ppm or more, 1% or more, or 10% or more. The firstheat treatment may be performed under a reduced pressure. Alternatively,the first heat treatment may be performed in such a manner that heattreatment is performed in an inert gas atmosphere, and then another heattreatment is performed in an atmosphere containing an oxidizing gas at10 ppm or more, 1% or more, or 10% or more in order to compensatedesorbed oxygen. By the first heat treatment, crystallinity of thesemiconductor can be increased and impurities such as hydrogen andmoisture can be removed, for example. Alternatively, in the first heattreatment, plasma treatment using oxygen may be performed under areduced pressure. The plasma treatment containing oxygen is preferablyperformed using an apparatus including a power source for generatinghigh-density plasma using microwaves, for example. Alternatively, aplasma power source for applying a radio frequency (RF) voltage to asubstrate side may be provided. The use of high-density plasma enableshigh-density oxygen radicals to be produced, and application of the RFvoltage to the substrate side allows oxygen radicals generated by thehigh-density plasma to be efficiently introduced into the semiconductor406 b. Alternatively, after plasma treatment using an inert gas with theapparatus, plasma treatment using oxygen in order to compensate releasedoxygen may be performed.

Next, the insulator to be the insulator 406 a and the semiconductor tobe the semiconductor 406 b are processed by a photolithography method orthe like to form a multilayer film including the insulator 406 a and thesemiconductor 406 b (see FIGS. 13A to 13C). Note that when themultilayer film is formed, the insulator 402 is also subjected etchingto have a thinned region in some cases. That is, the insulator 402 mayhave a protruding portion in a region in contact with the multilayerfilm.

Next, a conductor is formed. The conductor can be formed by a sputteringmethod, a CVD method, an MBE method, a PLD method, an ALD method, or thelike.

Then, a first resist is formed over the conductor by a photolithographymethod or the like. First etching is performed by dry etching or thelike using the first resist as a mask. Next, second etching isperformed. In the second etching, dry etching is performed on the firstresist using oxygen plasma or the like to reduce the first resist, sothat the second resist is formed. Then, the third etching is performed.In the third etching, the conductor is etched using the second resist asa mask to form the conductors 416 a 1 and 416 a 2 each having astep-like end portion. In the first etching, 20% to 80%, preferably 30%to 60%, of the thickness of the conductor is etched, whereby the endportions of the conductor can be thin. In this manner, the conductors416 a 1 and 416 a 2 each have regions with different thicknesses (seeFIGS. 14A to 14C).

Note that the conductors 416 a 1 and 416 a 2 cover the multilayer film.The side surface of the insulator 406 a and the top and side surfaces ofthe semiconductor 406 b are partly damaged in forming the conductorsover the multilayer film, and then a region where resistance is reducedmight be formed. Since each of the insulator 406 a and the semiconductor406 b includes a region whose resistance is lowered, the contactresistance between the semiconductor 406 b and the conductors 416 a 1and 416 a 2 can be lowered.

Next, a conductor is formed. The conductor can be formed by a sputteringmethod, a CVD method, an MBE method, a PLD method, an ALD method, or thelike. The conductor is processed by a photolithography method or thelike to form a conductor 418 (see FIGS. 15A to 15C).

Next, an insulator to be the insulator 410 is formed. The insulator tobe the insulator 410 can be formed by a sputtering method, a CVD method,an MBE method, a PLD method, an ALD method, or the like. Alternatively,the insulator to be the insulator 410 can be formed by a spin coatingmethod, a dipping method, a droplet discharging method (such as anink-jet method), a printing method (such as screen printing or offsetprinting), a doctor knife method, a roll coater method, a curtain coatermethod, or the like.

The insulator to be the insulator 410 may be formed to have a flat topsurface. For example, the top surface of the insulator to be theinsulator 410 may have planarity immediately after the film formation.Alternatively, the insulator to be the insulator 410 may be planarizedby removing the insulator or the like from the top surface so that thetop surface becomes parallel to a reference surface such as a rearsurface of the substrate. Such treatment is referred to as planarizationtreatment. As the planarization treatment, for example, chemicalmechanical polishing treatment, dry etching treatment, or the like canbe performed. However, the top surface of the insulator to be theinsulator 410 is not necessarily flat.

Next, a resist mask is formed over the insulator to be the insulator 410by a photolithography method or the like. Here, an organic coating filmmay be formed between the top surface of the insulator to be theinsulator 410 and the resist mask in order to improve the adhesionbetween the top surface of the insulator to be the insulator 410 and theresist mask.

Next, the insulator to be the insulator 410 is subjected to a firstprocessing by a dry etching method or the like to expose the top surfaceof the conductor 418. As a dry etching apparatus, any of the above dryetching apparatuses can be used; however, a dry etching apparatus inwhich high-frequency power sources with different frequencies areconnected to the parallel-plate electrodes is preferably used.

Then, the conductor 418 is subjected to a second processing by a dryetching method or the like so as to be separated into a conductive layerincluding the conductors 416 a 1 and 416 b 1 and a conductive layerincluding the conductors 416 a 2 and 416 b 2. Note that the insulator410 and the conductor 418 may be processed in the same photolithographyprocess. Processing in the same photolithography process can reduce thenumber of manufacturing steps. Thus, a semiconductor device includingthe transistor can be manufactured with high productivity.Alternatively, the insulator 410 and the conductor 418 may be processedin different photolithography processes. Processing in differentphotolithography processes may facilitate formation of films withdifferent shapes (see FIGS. 16A to 16C).

At this time, the semiconductor 406 b has a region that is exposed. Theexposed region of the semiconductor 406 b is partly removed by thesecond processing in some cases. Furthermore, impurity elements such asresidual components of the etching gas are attached to the exposedsurface of the semiconductor 406 b in some cases. For example, when achlorine-based gas is used as an etching gas, chlorine and the like areattached in some cases. Furthermore, when a hydrocarbon-based gas isused as an etching gas, carbon, hydrogen, and the like are attached insome cases. Thus, the impurity elements attached to the exposed surfaceof the semiconductor 406 b are preferably reduced. The impurity elementscan be reduced by cleaning treatment using dilute hydrofluoric acid,cleaning treatment using ozone, cleaning treatment using ultra violetrays, or the like. Note that some kinds of cleaning treatment may beused in combination. Accordingly, the exposed surface of thesemiconductor 406 b, that is, the channel formation region has a highresistance.

The method for forming the conductors 416 b 1 and 416 b 2 may bedifferent from the above method. For example, a conductor to be theconductors 416 b 1 and 416 b 2 is formed after the conductors 416 a 1and 416 a 2 are formed. Next, the conductor is processed by aphotolithography method or the like to form the conductors 416 b 1 and416 b 2. Then, the insulator to be the insulator 410 is formed andprocessed to have an opening through which the semiconductor 406 b isexposed.

Next, an insulator 407 a is formed (see FIGS. 17B and 17C). Theinsulator 407 a can be formed by a sputtering method, a CVD method, anMBE method, a PLD method, an ALD method, or the like. The insulator 407a may be formed in a manner similar to that of the insulator 406 a.

Next, the insulator 407 a is etched to form the insulator 406 c (seeFIGS. 18B and 18C). The etching is preferably performed by wet etching.Accordingly, the insulator 407 a can be selectively etched in accordancewith the crystallinity or the like of the insulator. As an etchant ofthe wet etching, an acid solution is preferably used. For example, asolution containing hydrofluoric acid, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, a solution containingoxalic acid, or a solution containing phosphoric acid can be used.Furthermore, the etchant is not limited to an acid solution and may bean alkaline solution. As the alkaline solution, an ammonia hydrogenperoxide mixture (a mixed solution of ammonia, water, and a hydrogenperoxide solution) may be used, for example.

In the insulator 407 a, a region in contact with the semiconductor 406 band the insulator 406 a has high crystallinity. In contrast, in theinsulator 407 a, a region in contact with the insulator 410 has lowcrystallinity. In particular, it is preferable that the insulator 406 aand the semiconductor 406 b each have a CAAC-OS, and thus the region ofthe insulator 407 a in contact with the insulator 406 a and thesemiconductor 406 b tends to be an insulator having a CAAC-OS. Theetching resistance of the region having a CAAC-OS is higher than that ofthe region not having a CAAC-OS.

Accordingly, since the insulator 407 a has regions with differentcrystallinities, the insulator 407 a can be selectively etched withoutusing a mask.

Then, an insulator 407 b is formed (see FIGS. 19B and 19C). Theinsulator 407 b can be formed by a sputtering method, a CVD method, anMBE method, a PLD method, an ALD method, or the like. The insulator 407b may be formed in a manner similar to that of the insulator 406 a.

Next, the insulator 407 b is etched to form the insulator 406 d (seeFIGS. 20B and 20C). The etching is preferably performed by wet etching.Accordingly, the insulator 407 b can be selectively etched in accordancewith the crystallinity or the like of the insulator. As an etchant ofthe wet etching, an acid solution is preferably used. For example, asolution containing hydrofluoric acid, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, a solution containingoxalic acid, or a solution containing phosphoric acid can be used.Furthermore, the etchant is not limited to an acid solution and may bean alkaline solution. As the alkaline solution, an ammonia hydrogenperoxide mixture (a mixed solution of ammonia, water, and a hydrogenperoxide solution) may be used, for example.

In the insulator 407 b, a region in contact with the insulator 406 c hashigh crystallinity. In contrast, in the insulator 407 b, a region incontact with the insulator 410 has low crystallinity. In particular, itis preferable that the insulator 406 c have a CAAC-OS, and thus theregion of the insulator 407 b in contact with the insulator 406 c tendsto be an insulator having a CAAC-OS. The etching resistance of theregion having a CAAC-OS is higher than that of the region not having aCAAC-OS. Accordingly, since the insulator 407 b has regions withdifferent crystallinities, the insulator 407 b can be selectively etchedwithout using a mask.

Next, an insulator to be the insulator 412 is formed over the insulators410 and 406 d. The insulator to be the insulator 412 can be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like.

Next, a conductor to be the conductor 404 is formed. The conductor to bethe conductor 404 can be formed by a sputtering method, a CVD method, anMBE method, a PLD method, an ALD method, or the like. The conductor tobe the conductor 404 is formed so as to fill the opening formed by theinsulator 410 and the like. Therefore, a CVD method (an MCVD method, inparticular) is preferred. A stacked-layer film of a conductor formed byan ALD method or the like and a conductor formed by a CVD method ispreferred in some cases to increase adhesion of the conductor formed byan MCVD method. For example, a stacked-layer film where titanium nitrideand tungsten are formed in this order may be used.

Then, the conductor to be the conductor 404 is processed by aphotolithography method to form the conductor 404.

Next, the insulator to be the insulator 412 is processed by aphotolithography method or the like, so that the insulator 412 is formed(see FIGS. 21B and 21C). Note that the conductor to be the conductor 404and the insulator to be the insulator 412 may be processed in the samephotolithography step. Processing in the same photolithography processcan reduce the number of manufacturing steps. Thus, productivity of asemiconductor device including a transistor can be increased.Alternatively, the conductor to be the conductor 404 and the insulatorto be the insulator 412 may be processed in different photolithographyprocesses. Processing in different photolithography processes mayfacilitate formation of films with different shapes. Although an examplewhere the insulator to be the insulator 412 is processed is shown here,the transistor of one embodiment of the present invention is not limitedthereto. For example, the insulator to be the insulator 412 may be usedwithout being processed in some cases.

Next, the insulator 408 is formed over the insulator 410 and theconductor 404. The insulator 408 can be formed by a sputtering method, aCVD method, an MBE method, a PLD method, an ALD method, or the like.Aluminum oxide is preferably formed as the insulator 408 using plasmacontaining oxygen, so that oxygen in the plasma can be added to the topsurface of the insulator 410 and a region of the insulator 412 incontact with the insulator 408 as excess oxygen (exO). Here, the mixedregion 414 containing a large amount of excess oxygen might be formed inthe interface between the insulator 408 and the insulator 410 and thevicinity of the interface. FIGS. 22A and 22B illustrate the state wherethe excess oxygen is added to the vicinity of the mixed region 414 byarrows.

Second heat treatment may be performed at any time after the formationof the insulator 408. By the second heat treatment, the excess oxygencontained in the insulator 410 and the mixed region 414 is moved to thesemiconductor 406 b through the insulator 412, the insulator 402, theinsulator 406 d, the insulator 406 c, and the insulator 406 a. Sinceexcess oxygen is moved to the semiconductor 406 b as described above,defects (oxygen vacancies) in the semiconductor 406 b can be reduced(see FIGS. 22C and 22D).

Note that the second heat treatment may be performed at a temperaturesuch that excess oxygen in the insulator 410 and the mixed region 414 isdiffused to the semiconductor 406 b. For example, the description of thefirst heat treatment may be referred to for the second heat treatment.Alternatively, the temperature of the second heat treatment ispreferably lower than that of the first heat treatment. The second heattreatment is preferably performed at a temperature lower than that ofthe first heat treatment by higher than or equal to 20° C. and lowerthan or equal to 150° C., preferably higher than or equal to 40° C. andlower than or equal to 100° C. Accordingly, superfluous release ofexcess oxygen from the insulator 402 or the like can be inhibited. Notethat the second heat treatment is not necessarily performed when heatingduring formation of the films can work as heat treatment comparable tothe second heat treatment.

Although not illustrated, an opening reaching the conductor 416 a 1 andan opening reaching the conductor 416 a 2 may be formed in the insulator408 and the insulator 410, and conductors serving as wirings may beformed in the openings. Alternatively, an opening reaching the conductor404 may be formed in the insulator 408, and a conductor serving as awiring may be formed in the opening.

Through the above steps, the transistor illustrated in FIGS. 1A to 1Ccan be manufactured.

In Embodiment 1, one embodiment of the present invention has beendescribed. Note that one embodiment of the present invention is notlimited to the above examples. That is, since various embodiments of thepresent invention are disclosed in this embodiment and otherembodiments, one embodiment of the present invention is not limited to aspecific embodiment. The example in which an oxide semiconductor is usedas a semiconductor has been described as one embodiment of the presentinvention; however, one embodiment of the present invention is notlimited thereto. Depending on cases or conditions, silicon, germanium,silicon germanium, silicon carbide, gallium arsenide, aluminum galliumarsenide, indium phosphide, gallium nitride, an organic semiconductor,or the like may be used in one embodiment of the present invention.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2 Transistor Structure 2

A transistor having a structure different from that in FIGS. 1A to 1Cand a manufacturing method thereof will be described with reference toFIGS. 23A to 23C and FIGS. 24A to 24C. FIGS. 23A to 23C are a top viewand cross-sectional views of a semiconductor device of one embodiment ofthe present invention. FIG. 23A is the top view, and FIGS. 23B and 23Care the cross-sectional views taken along dashed-dotted lines A1-A2 andA3-A4 in FIG. 23A, respectively. Note that for simplification of thedrawing, some components are not illustrated in the top view in FIG.23A.

In the transistor illustrated in FIGS. 23A to 23C, conductors 416 a 3and 416 a 4 serving as a source electrode and a drain electrode areformed only over the top surface of the semiconductor 406 b. For theother components, the description of the transistor in FIGS. 1A to 1C isreferred to.

<Method 2 for Manufacturing Transistor>

A method for manufacturing the transistor illustrated in FIGS. 23A to23C will be described below with reference to FIGS. 24A to 24C.

First, the substrate 400 is prepared. Next, a conductor is formed overthe substrate 400 and is then processed by a photolithography method orthe like to form the conductor 413. Next, the insulator 402 is formed.

Next, the insulator to be the insulator 406 a is formed over theinsulator 402. Next, the semiconductor to be the semiconductor 406 b isformed over the insulator to be the insulator 406 a.

Next, first heat treatment is preferably performed. The first heattreatment may be performed in a manner similar to that in Embodiment 1.Alternatively, in the first heat treatment, plasma treatment usingoxygen may be performed under a reduced pressure.

Next, a conductor to be a conductor 419 is formed over the semiconductorto be the semiconductor 406 b. The conductor can be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. Then, the insulator to be the insulator 406 a, thesemiconductor to be the semiconductor 406 b, and the conductor to be theconductor 419 are processed by a lithography method or the like to forma multilayer film including the insulator 406 a, the semiconductor 406b, and the conductor 419. Here, a top surface of the semiconductor to bethe semiconductor 406 b is damaged when the conductor is formed, wherebya region whose resistance is decreased is formed in some cases. Thesemiconductor 406 b includes the region whose resistance is decreased;thus, contact resistance between the conductor 419 and the semiconductor406 b is reduced. Note that when the multilayer film is formed, theinsulator 402 is also subjected etching to have a thinned region in somecases. That is, the insulator 402 may have a protruding portion in aregion in contact with the multilayer film (see FIGS. 24A to 24C).

The subsequent steps may be performed in a manner similar to the stepsillustrated in FIGS. 16A to 16C, FIGS. 17A to 17C, FIGS. 18A to 18C,FIGS. 19A to 19C, FIGS. 20A to 20C, FIGS. 21A to 21C, and FIGS. 22A to22C in the method 1 for manufacturing the transistor described inEmbodiment 1.

Through the above steps, the transistor illustrated in FIGS. 23A to 23Ccan be manufactured.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3 Transistor Structure 3

A transistor having a structure different from that in FIGS. 1A to 1Cand a manufacturing method thereof will be described with reference toFIGS. 25A to 25C. FIGS. 25A to 25C are a top view and cross-sectionalviews of a semiconductor device of one embodiment of the presentinvention. FIG. 25A is the top view, and FIGS. 25B and 25C are thecross-sectional views taken along dashed-dotted lines A1-A2 and A3-A4 inFIG. 25A, respectively. Note that for simplification of the drawing,some components are not illustrated in the top view in FIG. 25A.

In the transistor illustrated in FIGS. 25A to 25C, a conductor 411serving as the gate electrode can be formed in a self-aligned manner byCMP treatment or the like without using a photolithography method. Forthe other components, the description of the transistor in FIGS. 1A to1C is referred to.

<Method 3 for Manufacturing Transistor>

A method for manufacturing the transistor illustrated in FIGS. 25A to25C is described below.

First, the steps up to the step which is illustrated in FIGS. 20A to 20Cand described in Embodiment 1 are performed.

Next, an insulator to be an insulator 415 and a conductor to be aconductor 411 are formed, and then CMP treatment is performed so that asurface of the insulator 410 is exposed, whereby the insulator 415 andthe conductor 411 are formed.

Accordingly, the conductor 411 serving as the gate electrode can beformed in a self-aligned manner without using a lithography method. Theconductor 411 serving as the gate electrode can be formed withoutconsidering alignment accuracy of the conductor 411 serving as the gateelectrode and the conductors 416 a 1 and 416 a 2 serving as the sourceelectrode and the drain electrode; as a result, the area of thesemiconductor device can be reduced. Furthermore, because thelithography process is not necessary, improvement in productivity due tosimplification of the process is expected.

The insulator 410 serves as a stopper in CMP treatment. CMP treatment isnot necessarily performed until the surface of the insulator 410 isexposed and may be performed until the surface of the insulator 415 isexposed, whereby the conductor 411 is formed. In addition, the insulator415 may include an insulator serving as a stopper in CMP treatment.

Next, the insulator 408 is formed over the conductor 411 and theinsulator 410.

Through the above steps, the transistor illustrated in FIGS. 25A to 25Ccan be manufactured.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 4 Transistor Structure 4

A transistor having a structure different from that in FIGS. 1A to 1Cand a manufacturing method thereof will be described with reference toFIGS. 26A to 26C, FIGS. 27A to 27C, and FIGS. 28A to 28C. FIGS. 26A to26C are a top view and cross-sectional views of a semiconductor deviceof one embodiment of the present invention. FIG. 26A is the top view,and FIGS. 26B and 26C are the cross-sectional views taken alongdashed-dotted lines A1-A2 and A3-A4 in FIG. 23A, respectively. Note thatfor simplification of the drawing, some components are not illustratedin the top view of FIG. 26A.

The transistor is different from the transistor in FIGS. 1A to 1C inthat the shapes of the insulators 406 c and 406 d are different. For theother components, the description of the transistor in FIGS. 1A to 1C isreferred to.

<Method 4 for Manufacturing Transistor>

A method for manufacturing the transistor illustrated in FIGS. 26A to26C is described below with reference to FIGS. 27A to 27C and FIGS. 28Ato 28C.

First, the steps up to the step in FIG. 16C in Embodiment 1 areperformed.

Next, the insulator 407 a is formed, and then, the insulator 407 b isformed. The insulators 407 a and 407 b may be formed as in Embodiment 1(see FIGS. 27B and 27C).

Next, the insulator 407 a and the insulator 407 b are etched to form theinsulator 406 c and the insulator 406 d (see FIGS. 28B and 28C). Theetching is preferably performed by wet etching. Accordingly, theinsulator 407 a and the insulator 407 b can be selectively etchedwithout using a mask in accordance with the crystallinity or the like ofthe insulators. As an etchant of the wet etching, an acid solution ispreferably used. For example, a solution containing hydrofluoric acid, asolution obtained by mixing phosphoric acid, acetic acid, and nitricacid, a solution containing oxalic acid, or a solution containingphosphoric acid can be used. Furthermore, the etchant is not limited toan acid solution and may be an alkaline solution. As the alkalinesolution, an ammonia hydrogen peroxide mixture (a mixed solution ofammonia, water, and a hydrogen peroxide solution) may be used, forexample.

The subsequent steps may be performed in a manner similar to the stepsillustrated in FIGS. 21A to 21C and FIGS. 22A to 22D in the method 1 formanufacturing the transistor described in Embodiment 1.

Through the above steps, the transistor illustrated in FIGS. 26A to 26Ccan be manufactured.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 5 Memory Device 1

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 29A and 29B.

The semiconductor device illustrated in FIG. 29A includes a transistor3200 using a first semiconductor, a transistor 3300 using a secondsemiconductor, and a capacitor 3400. Note that any of theabove-described transistors can be used as the transistor 3300.

Note that the transistor 3300 is preferably a transistor with a lowoff-state current. For example, a transistor using an oxidesemiconductor can be used as the transistor 3300. Since the off-statecurrent of the transistor 3300 is low, stored data can be retained for along period at a predetermined node of the semiconductor device. Inother words, power consumption of the semiconductor device can bereduced because refresh operation becomes unnecessary or the frequencyof refresh operation can be extremely low.

In FIG. 29A, a first wiring 3001 is electrically connected to a sourceof the transistor 3200. A second wiring 3002 is electrically connectedto a drain of the transistor 3200. A third wiring 3003 is electricallyconnected to one of the source and the drain of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate of thetransistor 3300. The gate of the transistor 3200 and the other of thesource and the drain of the transistor 3300 are electrically connectedto a first terminal of the capacitor 3400. A fifth wiring 3005 iselectrically connected to a second terminal of the capacitor 3400.

The semiconductor device in FIG. 29A has a feature that the potential ofthe gate of the transistor 3200 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 ison, so that the transistor 3300 is turned on. Accordingly, the potentialof the third wiring 3003 is supplied to a node FG where the gate of thetransistor 3200 and the first terminal of the capacitor 3400 areelectrically connected to each other. That is, a predetermined electriccharge is supplied to the gate of the transistor 3200 (writing). Here,one of two kinds of electric charges providing different potentiallevels (hereinafter referred to as a low-level electric charge and ahigh-level electric charge) is supplied. After that, the potential ofthe fourth wiring 3004 is set to a potential at which the transistor3300 is off, so that the transistor 3300 is turned off. Thus, theelectric charge is held at the node FG (retaining).

Since the off-state current of the transistor 3300 is low, the electriccharge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of electric charge retained in the node FG. This is because inthe case of using an n-channel transistor as the transistor 3200, anapparent threshold voltage V_(th_H) at the time when the high-levelelectric charge is given to the gate of the transistor 3200 is lowerthan an apparent threshold voltage V_(th_L) at the time when thelow-level electric charge is given to the gate of the transistor 3200.Here, an apparent threshold voltage refers to the potential of the fifthwiring 3005 which is needed to make the transistor 3200 be in “onstate”. Thus, the potential of the fifth wiring 3005 is set to apotential V₀ which is between V_(th_H) and V_(th_L), whereby electriccharge supplied to the node FG can be determined. For example, in thecase where the high-level electric charge is supplied to the node FG inwriting and the potential of the fifth wiring 3005 is V₀ (>V_(th_H)),the transistor 3200 is brought into “on state”. In the case where thelow-level electric charge is supplied to the node FG in writing, evenwhen the potential of the fifth wiring 3005 is V₀ (<V_(th_L)), thetransistor 3200 still remains in “off state”. Thus, the data retained inthe node FG can be read by determining the potential of the secondwiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell be read in read operation. The fifthwiring 3005 of memory cells from which data is not read may be suppliedwith a potential at which the transistor 3200 is turned off regardlessof the electric charge supplied to the node FG, that is, a potentiallower than V_(th_H), whereby only data of a desired memory cell can beread. Alternatively, the fifth wiring 3005 of the memory cells fromwhich data is not read may be supplied with a potential at which thetransistor 3200 is brought into “on state” regardless of the electriccharge supplied to the node FG, that is, a potential higher than V_(th)_(L) , whereby only data of a desired memory cell can be read.

<Structure 1 of Semiconductor Device>

FIG. 30 is a cross-sectional view of the semiconductor device in FIG.29A. The semiconductor device shown in FIG. 30 includes the transistor3200, the transistor 3300, and the capacitor 3400. The transistor 3300and the capacitor 3400 are provided over the transistor 3200. Althoughan example where the transistor illustrated in FIGS. 1A to 1C is used asthe transistor 3300 is shown, a semiconductor device of one embodimentof the present invention is not limited thereto. The description of theabove transistor is referred to.

The transistor 3200 illustrated in FIG. 30 is a transistor using asemiconductor substrate 450. The transistor 3200 includes a region 474 ain the semiconductor substrate 450, a region 474 b in the semiconductorsubstrate 450, an insulator 462, and a conductor 454.

In the transistor 3200, the regions 474 a and 474 b have a function as asource region and a drain region. The insulator 462 has a function as agate insulator. The conductor 454 has a function as a gate electrode.Therefore, resistance of a channel formation region can be controlled bya potential applied to the conductor 454. In other words, conduction ornon-conduction between the region 474 a and the region 474 b can becontrolled by the potential applied to the conductor 454.

For the semiconductor substrate 450, a single-material semiconductorsubstrate of silicon, germanium, or the like or a compound semiconductorsubstrate of silicon carbide, silicon germanium, gallium arsenide,indium phosphide, zinc oxide, gallium oxide, or the like may be used,for example. A single crystal silicon substrate is preferably used asthe semiconductor substrate 450.

For the semiconductor substrate 450, a semiconductor substrate includingimpurities imparting n-type conductivity is used. However, asemiconductor substrate including impurities imparting p-typeconductivity may be used as the semiconductor substrate 450. In thatcase, a well including impurities imparting the n-type conductivity maybe provided in a region where the transistor 3200 is formed.Alternatively, the semiconductor substrate 450 may be an i-typesemiconductor substrate.

The top surface of the semiconductor substrate 450 preferably has a(110) plane. Thus, on-state characteristics of the transistor 3200 canbe improved.

The regions 474 a and 474 b are regions including impurities impartingthe p-type conductivity. Accordingly, the transistor 3200 has astructure of a p-channel transistor.

Note that although the transistor 3200 is illustrated as a p-channeltransistor, the transistor 3200 may be an n-channel transistor.

Note that the transistor 3200 is separated from an adjacent transistorby the region 460 and the like. The region 460 is an insulating region.

The semiconductor illustrated in FIG. 30 includes an insulator 464, aninsulator 466, an insulator 468, an insulator 470, an insulator 472, aninsulator 475, the insulator 402, the insulator 410, the insulator 408,an insulator 428, an insulator 465, an insulator 467, an insulator 469,an insulator 498, a conductor 480 a, a conductor 480 b, a conductor 480c, a conductor 478 a, a conductor 478 b, a conductor 478 c, a conductor476 a, a conductor 476 b, a conductor 476 c, a conductor 479 a, aconductor 479 b, a conductor 479 c, a conductor 477 a, a conductor 477b, a conductor 477 c, a conductor 484 a, a conductor 484 b, a conductor484 c, a conductor 484 d, a conductor 483 a, a conductor 483 b, aconductor 483 c, a conductor 483 d, a conductor 485 a, a conductor 485b, a conductor 485 c, a conductor 485 d, a conductor 487 a, a conductor487 b, a conductor 487 c, a conductor 488 a, a conductor 488 b, aconductor 488 c, a conductor 490 a, a conductor 490 b, a conductor 489a, a conductor 489 b, a conductor 491 a, a conductor 491 b, a conductor491 c, a conductor 492 a, a conductor 492 b, a conductor 492 c, aconductor 494, a conductor 496, an insulator 406 a, a semiconductor 406b, and an insulator 406 c.

The insulator 464 is provided over the transistor 3200. The insulator466 is over the insulator 464. The insulator 468 is over the insulator466. The insulator 470 is placed over the insulator 468. The insulator472 is placed over the insulator 470. The insulator 475 is placed overthe insulator 472. The transistor 3300 is provided over the insulator475. The insulator 408 is provided over the transistor 3300. Theinsulator 428 is provided over the insulator 408. The insulator 465 isover the insulator 428. The capacitor 3400 is provided over theinsulator 465. The insulator 469 is provided over the capacitor 3400.

The insulator 464 includes an opening reaching the region 474 a, anopening reaching the region 474 b, and an opening reaching the conductor454, in which the conductor 480 a, the conductor 480 b, and theconductor 480 c are embedded, respectively.

In addition, the insulator 466 includes an opening reaching theconductor 480 a, an opening reaching the conductor 480 b, and an openingreaching the conductor 480 c, in which the conductor 478 a, theconductor 478 b, and the conductor 478 c are embedded, respectively.

In addition, the insulator 468 includes an opening reaching theconductor 478 a, an opening reaching the conductor 478 b, and an openingreaching the conductor 478 c, in which the conductor 476 a, theconductor 476 b, and the conductor 476 c are embedded, respectively.

The conductor 479 a in contact with the conductor 476 a, the conductor479 b in contact with the conductor 476 b, and the conductor 479 c incontact with the conductor 476 c are over the insulator 468. Theinsulator 472 includes an opening reaching the conductor 479 a throughthe insulator 470 and an opening reaching the conductor 479 b throughthe insulator 470. In the openings, the conductor 477 a and theconductor 477 b are embedded.

The insulator 475 includes an opening overlapping with the channelformation region of the transistor 3300, an opening reaching theconductor 477 a, an opening reaching the conductor 477 b, and an openingreaching the insulator 472. In the openings, the conductor 484 a, theconductor 484 b, the conductor 484 c, and the conductor 484 d isembedded.

The conductor 484 d may have a function as a bottom gate electrode ofthe transistor 3300. Alternatively, for example, electriccharacteristics such as the threshold voltage of the transistor 3300 maybe controlled by application of a constant potential to the conductor484 d. Further alternatively, for example, the conductor 484 d and thetop gate electrode of the transistor 3300 may be electrically connectedto each other. Thus, the on-state current of the transistor 3300 can beincreased. A punch-through phenomenon can be suppressed; thus, stableelectric characteristics in the saturation region of the transistor 3300can be obtained.

In addition, the insulator 402 includes an opening reaching theconductor 484 a and an opening reaching the conductor 484 c.

The insulator 428 includes two openings reaching the conductor 484 athrough the insulator 408, the insulator 410, and the insulator 402, twoopenings reaching a conductor of one of the source electrode and thedrain electrode of the transistor 3300 through the insulator 408 and theinsulator 410, and an opening reaching a conductor of the gate electrodeof the transistor 3300 through the insulator 408. In the openings, theconductor 483 a, the conductor 483 c, and the conductor 483 d areembedded.

The conductor 485 a in contact with the conductor 483 a, the conductor485 b in contact with the conductor 483 b, the conductor 485 c incontact with the conductor 483 c, and the conductor 485 d in contactwith the conductor 483 d are over the insulator 428. The insulator 465has an opening reaching the conductor 485 a, an opening reaching theconductor 485 b, and an opening reaching the conductor 485 c. In theopenings, the conductor 487 a, the conductor 487 b, and the conductor487 c are embedded.

The conductor 488 a in contact with the conductor 487 a, the conductor488 b in contact with the conductor 487 b, and the conductor 488 c incontact with the conductor 487 c are over the insulator 465. Inaddition, the insulator 467 includes an opening reaching the conductor488 a and an opening reaching the conductor 488 b. In the openings, theconductor 490 a and the conductor 490 b are embedded. The conductor 488c is in contact with the conductor 494 which is the first terminal ofthe capacitor 3400.

The conductor 489 a in contact with the conductor 490 a and theconductor 489 b in contact with the conductor 490 b are over theinsulator 467. The insulator 469 includes an opening reaching theconductor 489 a, an opening reaching the conductor 489 b, an openingreaching the conductor 496 which is the second terminal of the capacitor3400. In the openings, the conductor 491 a, the conductor 491 b, and theconductor 491 c are embedded.

The conductor 492 a in contact with the conductor 491 a, the conductor492 b in contact with the conductor 491 b, and the conductor 492 c incontact with the conductor 491 c are over the insulator 469.

The insulators 464, 466, 468, 470, 472, 475, 402, 410, 408, 428, 465,467, 469, and 498 may each be formed to have, for example, asingle-layer structure or a stacked-layer structure including aninsulator containing boron, carbon, nitrogen, oxygen, fluorine,magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium,germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, ortantalum. The insulator 401 may be formed of, for example, aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,or tantalum oxide.

The insulator that has a function of blocking oxygen and impurities suchas hydrogen is preferably included in at least one of the insulators464, 466, 468, 470, 472, 475, 402, 410, 408, 428, 465, 467, 469, and498. When an insulator that has a function of blocking oxygen andimpurities such as hydrogen is placed near the transistor 3300, theelectric characteristics of the transistor 3300 can be stable.

An insulator with a function of blocking oxygen and impurities such ashydrogen may have a single-layer structure or a stacked-layer structureincluding an insulator containing, for example, boron, carbon, nitrogen,oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine,argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium,hafnium, or tantalum may be used.

Each of the conductors 480 a, 480 b, 480 c, 478 a, 478 b, 478 c, 476 a,476 b, 476 c, 479 a, 479 b, 479 c, 477 a, 477 b, 477 c, 484 a, 484 b,484 c, 484 d, 483 a, 483 b, 483 c, 483 d, 485 a, 485 b, 485 c, 485 d,487 a, 487 b, 487 c, 488 a, 488 b, 488 c, 490 a, 490 b, 489 a, 489 b,491 a, 491 b, 491 c, 492 a, 492 b, 492 c, 494, and 496 may have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. Analloy or a compound may be used, for example, and a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

An oxide semiconductor is preferably used as the semiconductor 406 b.However, silicon (including strained silicon), germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, an organic semiconductor, or the likecan be used in some cases.

As the insulator 406 a and the insulator 406 c, oxides containing one ormore, or two or more elements other than oxygen included in thesemiconductor 406 b are preferably used. However, silicon (includingstrained silicon), germanium, silicon germanium, silicon carbide,gallium arsenide, aluminum gallium arsenide, indium phosphide, galliumnitride, an organic semiconductor, or the like can be used in somecases.

The source or drain of the transistor 3200 is electrically connected tothe conductor that is one of the source electrode and the drainelectrode of the transistor 3300 through the conductor 480 a, theconductor 478 a, the conductor 476 a, the conductor 479 a, the conductor477 a, the conductor 484 a, the conductor 483 a, and the conductor 485a. The conductor 454 that is the gate electrode of the transistor 3200is electrically connected to the conductor that is the other of thesource electrode and the drain electrode of the transistor 3300 throughthe conductor 480 c, the conductor 478 c, the conductor 476 c, theconductor 479 c, the conductor 477 c, the conductor 484 c, the conductor483 c, and the conductor 485 c.

The capacitor 3400 includes one of the source electrode and the drainelectrode of the transistor 3300, the conductor 494 electricallyconnected to the first terminal of the capacitor 3400 through theconductor 483 c, the conductor 485 c, the conductor 487 c, and theconductor 488 c, the insulator 498, the conductor 496 which is thesecond terminal of the capacitor 3400. The capacitor 3400 is preferablyformed above or below the transistor 3300 because the semiconductor canbe reduced in size.

For the structures of other components, the description of FIGS. 1A to1C and the like can be referred to as appropriate.

A semiconductor device in FIG. 31 is the same as the semiconductordevice in FIG. 30 except the structure of the transistor 3200.Therefore, the description of the semiconductor device in FIG. 30 isreferred to for the semiconductor device in FIG. 31. Specifically, inthe semiconductor device in FIG. 31, the transistor 3200 is a FIN-typetransistor. The effective channel width is increased in the FIN-typetransistor 3200, whereby the on-state characteristics of the transistor3200 can be improved. In addition, since contribution of the electricfield of the gate electrode can be increased, the off-statecharacteristics of the transistor 3200 can be improved. Note that thetransistor 3200 may be a p-channel transistor or an n-channeltransistor.

Although an example in which the transistor 3300 is over the transistor3200 and the capacitor 3400 is over the transistor 3300 is illustratedin this embodiment, one or more transistors including a semiconductorsimilar to the transistor 3300 may be provided over the transistor 3200.With such a structure, the degree of integration of the semiconductordevice can be further increased.

<Memory Device 2>

The semiconductor device in FIG. 29B is different from the semiconductordevice in FIG. 29A in that the transistor 3200 is not provided. Also inthis case, data can be written and retained in a manner similar to thatof the semiconductor device in FIG. 29A.

Reading of data in the semiconductor device in FIG. 29B is described.When the transistor 3300 is turned on, the third wiring 3003 which is ina floating state and the capacitor 3400 are electrically connected toeach other, and the charge is redistributed between the third wiring3003 and the capacitor 3400. As a result, the potential of the thirdwiring 3003 is changed. The amount of change in potential of the thirdwiring 3003 varies depending on the potential of the first terminal ofthe capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof first terminal of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of thefirst terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potentialof the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀) (C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having anlow off-state current, the semiconductor device described above canretain stored data for a long time. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device, high voltage is not needed forwriting data and deterioration of elements is less likely to occur.Unlike in a conventional nonvolatile memory, for example, it is notnecessary to inject and extract electrons into and from a floating gate;thus, a problem such as deterioration of an insulator is not caused.That is, the semiconductor device of one embodiment of the presentinvention does not have a limit on the number of times data can berewritten, which is a problem of a conventional nonvolatile memory, andthe reliability thereof is drastically improved. Furthermore, data iswritten depending on the state of the transistor (on or off), wherebyhigh-speed operation can be easily achieved. At least part of thisembodiment can be implemented in combination with any of the embodimentsdescribed in this specification as appropriate.

Embodiment 6 Structure 2 of Semiconductor Device

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention is described with reference todrawings.

<Cross-Sectional Structure>

FIGS. 32A and 32B are cross-sectional views of a semiconductor device ofone embodiment of the present invention. In FIG. 32A, X1-X2 directionrepresents a channel length direction, and in FIG. 32B, Y1-Y2 directionrepresents a channel width direction. The semiconductor deviceillustrated in FIGS. 32A and 32B includes a transistor 2200 containing afirst semiconductor material in a lower portion and a transistor 2100containing a second semiconductor material in an upper portion. In FIGS.32A and 32B, an example is illustrated in which the transistorillustrated in FIGS. 1A to 1C is used as the transistor 2100 containingthe second semiconductor material.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor using an oxide semiconductor anddescribed in the above embodiment as an example has excellentsubthreshold characteristics and a minute structure. Furthermore, thetransistor can operate at a high speed because of its high switchingspeed and has low leakage current because of its low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIGS. 32A and 32B illustrate a structure in which the transistor 2100 isprovided over the transistor 2200 with an insulator 2201, an insulator2207, and an insulator 2208 provided therebetween. A plurality ofwirings 2202 are provided between the transistor 2200 and the transistor2100. Furthermore, wirings and electrodes provided in an upper portionin which the transistor 2100 is provided and those in a lower portion inwhich the transistor 2200 is provided are electrically connected to eachother through a plurality of plugs 2203 embedded in insulators. Aninsulator 2204 covering the transistor 2100 and a wiring 2205 over theinsulator 2204 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in the lower portion, hydrogen in aninsulator provided in the vicinity of the semiconductor film of thetransistor 2200 terminates dangling bonds of silicon; accordingly, thereliability of the transistor 2200 can be improved. Meanwhile, in thecase where an oxide semiconductor is used for the transistor 2100provided in the upper portion, hydrogen in an insulator provided in thevicinity of the semiconductor film of the transistor 2100 becomes afactor of generating carriers in the oxide semiconductor; thus, thereliability of the transistor 2100 might be decreased. Therefore, in thecase where the transistor 2100 using an oxide semiconductor is providedover the transistor 2200 using a silicon-based semiconductor material,it is particularly effective that the insulator 2207 having a functionof preventing diffusion of hydrogen is provided between the transistors2100 and 2200. The insulator 2207 makes hydrogen remain in the lowerportion, thereby improving the reliability of the transistor 2200. Inaddition, since the insulator 2207 suppresses diffusion of hydrogen fromthe lower portion to the upper portion, the reliability of thetransistor 2100 also can be improved.

The insulator 2207 can be, for example, formed using aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film having a function of preventing diffusionof hydrogen is preferably formed over the transistor 2100 to cover thetransistor 2100 including an oxide semiconductor film. For the blockingfilm, a material that is similar to that of the insulator 2207 can beused, and in particular, an aluminum oxide film is preferably used.Using the aluminum oxide film, excess oxygen can be added to theinsulator under the aluminum oxide film in the deposition, and theexcess oxygen moves to the oxide semiconductor layer of the transistor2100 by heat treatment, which has an effect of repairing a defect in theoxide semiconductor layer. The aluminum oxide film has a high shielding(blocking) effect of preventing penetration of both oxygen andimpurities such as hydrogen and moisture. Thus, by using the aluminumoxide film as the blocking film covering the transistor 2100, release ofoxygen from the oxide semiconductor film included in the transistor 2100and entry of water and hydrogen into the oxide semiconductor film can beprevented. Note that as the block film, the insulator 2204 having astacked-layer structure may be used, or the block film may be providedunder the insulator 2204.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. An example of a cross-sectional view in this case is shown inFIGS. 32E and 32F. An insulator 2212 is provided over a semiconductorsubstrate 2211. The semiconductor substrate 2211 includes a projectingportion with a thin tip (also referred to a fin). Note that an insulatormay be provided over the projecting portion. The insulator functions asa mask for preventing the semiconductor substrate 2211 from being etchedwhen the projecting portion is formed. The projecting portion does notnecessarily have the thin tip; a projecting portion with a cuboid-likeprojecting portion and a projecting portion with a thick tip arepermitted, for example. A gate insulator 2214 is provided over theprojecting portion of the semiconductor substrate 2211, and a gateelectrode 2213 is provided over the gate insulator 2214. Source anddrain regions 2215 are formed in the semiconductor substrate 2211. Notethat here is shown an example in which the semiconductor substrate 2211includes the projecting portion; however, a semiconductor device of oneembodiment of the present invention is not limited thereto. For example,a semiconductor region having a projecting portion may be formed byprocessing an SOI substrate.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 7 CMOS Circuit

A circuit diagram in FIG. 32C shows a configuration of a so-called CMOScircuit in which a p-channel transistor 2200 and an n-channel transistor2100 are connected to each other in series and in which gates of themare connected to each other.

[Analog Switch]

A circuit diagram in FIG. 32D shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as a so-called analogswitch. At least part of this embodiment can be implemented incombination with any of the embodiments described in this specificationas appropriate.

Embodiment 8 CPU

A CPU including a semiconductor device such as any of theabove-described transistors or the above-described memory device isdescribed below.

FIG. 33 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 33 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and an ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 33 isjust an example of a simplified structure, and an actual CPU may have avariety of structures depending on the application. For example, the CPUmay have the following configuration: a structure including the CPUillustrated in FIG. 33 or an arithmetic circuit is considered as onecore; a plurality of the cores are included; and the cores operate inparallel. The number of bits that the CPU can process in an internalarithmetic circuit or in a data bus can be 8, 16, 32, or 64, forexample.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

In the CPU illustrated in FIG. 33, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 33, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of thepower supply voltage to the memory cell in the register 1196 can bestopped.

FIG. 34 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the line, or thelike is actively utilized.

A control signal WE is input to a first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 34illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 34, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 34, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a film formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon film or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor. Further alternatively, in the memory element 1200, atransistor in which a channel is formed in an oxide semiconductor can beincluded besides the transistor 1209, and a transistor in which achannel is formed in a layer including a semiconductor other than anoxide semiconductor or the substrate 1190 can be used for the rest ofthe transistors.

As the circuit 1201 in FIG. 34, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By using the above-described memory element 1200 for a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Further, shortly after thesupply of the power supply voltage is restarted, the memory element canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency (RF) tag.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 9 Imaging Device

FIG. 35A is a top view illustrating an example of an imaging device 200of one embodiment of the present invention. The imaging device 200includes a pixel portion 210 and peripheral circuits for driving thepixel portion 210 (a peripheral circuit 260, a peripheral circuit 270, aperipheral circuit 280, and a peripheral circuit 290). The pixel portion210 includes a plurality of pixels 211 arranged in a matrix with p rowsand q columns (p and q are each a natural number greater than or equalto 2). The peripheral circuit 260, the peripheral circuit 270, theperipheral circuit 280, and the peripheral circuit 290 are eachconnected to a plurality of pixels 211, and a signal for driving theplurality of pixels 211 is supplied. In this specification and the like,in some cases, “a peripheral circuit” or “a driver circuit” indicatesall of the peripheral circuits 260, 270, 280, and 290. For example, theperipheral circuit 260 can be regarded as part of the peripheralcircuit.

The imaging device 200 preferably includes a light source 291. The lightsource 291 can emit detection light P1.

The peripheral circuit includes at least one of a logic circuit, aswitch, a buffer, an amplifier circuit, and a converter circuit. Theperipheral circuit may be formed over a substrate where the pixelportion 210 is formed. Alternatively, a semiconductor device such as anIC chip may be used as part or the whole of the peripheral circuit. Notethat as the peripheral circuit, one or more of the peripheral circuits260, 270, 280, and 290 may be omitted.

As illustrated in FIG. 35B, the pixels 211 may be provided to beinclined in the pixel portion 210 included in the imaging device 200.When the pixels 211 are obliquely arranged, the distance between pixels(pitch) can be shortened in the row direction and the column direction.Accordingly, the quality of an image taken with the imaging device 200can be improved.

<Configuration Example 1 of Pixel>

The pixel 211 included in the imaging device 200 is formed with aplurality of subpixels 212, and each subpixel 212 is combined with afilter which transmits light with a specific wavelength band (colorfilter), whereby data for achieving color image display can be obtained.

FIG. 36A is a top view showing an example of the pixel 211 with which acolor image is obtained. The pixel 211 illustrated in FIG. 36A includesthe subpixel 212 provided with a color filter that transmits light witha red (R) wavelength band (also referred to as a subpixel 212R), asubpixel 212 provided with a color filter that transmits light with agreen (G) wavelength band (also referred to as a subpixel 212G), and asubpixel 212 provided with a color filter that transmits light with ablue (B) wavelength band (also referred to as a subpixel 212B). Thesubpixel 212 can function as a photosensor.

The subpixel 212 (the subpixel 212R, the subpixel 212G, and the subpixel212B) is electrically connected to a wiring 231, a wiring 247, a wiring248, a wiring 249, and a wiring 250. In addition, the subpixel 212R, thesubpixel 212G, and the subpixel 212B are connected to respective wirings253 which are independent from one another. In this specification andthe like, for example, the wiring 248 and the wiring 249 that areconnected to the pixel 211 in the n-th row are referred to as a wiring248[n] and a wiring 249[n], respectively. For example, the wiring 253connected to the pixel 211 in the m-th column is referred to as a wiring253[m]. Note that in FIG. 36A, the wirings 253 connected to the subpixel212R, the subpixel 212G, and the subpixel 212B in the pixel 211 in them-th column are referred to as a wiring 253[m]R, a wiring 253[m]G, and awiring 253[m]B. The subpixels 212 are electrically connected to theperipheral circuit through the above wirings.

The imaging device 200 has a structure in which the subpixel 212 iselectrically connected to the subpixel 212 in an adjacent pixel 211 thatis provided with a color filter that transmits light with the samewavelength band as the subpixel 212, via a switch. FIG. 36B shows aconnection example of the subpixels 212: the subpixel 212 in the pixel211 arranged in an n-th (n is an integer greater than or equal to 1 andless than or equal to p) row and an m-th (m is an integer greater thanor equal to 1 and less than or equal to q) column and the subpixel 212in the adjacent pixel 211 arranged in an (n+1)-th row and the m-thcolumn. In FIG. 36B, the subpixel 212R arranged in the n-th row and them-th column and the subpixel 212R arranged in the (n+1)-th row and them-th column are connected to each other via a switch 201. The subpixel212G arranged in the n-th row and the m-th column and the subpixel 212Garranged in the (n+1)-th row and the m-th column are connected to eachother via a switch 202. The subpixel 212B arranged in the n-th row andthe m-th column and the subpixel 212B arranged in the (n+1)-th row andthe m-th column are connected to each other via a switch 203.

The color filter used in the subpixel 212 is not limited to red (R),green (G), and blue (B) color filters, and color filters that transmitlight of cyan (C), yellow (Y), and magenta (M) may be used. By provisionof the subpixels 212 that sense light with three different wavelengthbands in one pixel 211, a full-color image can be obtained.

The pixel 211 including the subpixel 212 provided with a color filtertransmitting yellow (Y) light may be provided, in addition to thesubpixels 212 provided with the color filters transmitting red (R),green (G), and blue (B) light. The pixel 211 including the subpixel 212provided with a color filter transmitting blue (B) light may beprovided, in addition to the subpixels 212 provided with the colorfilters transmitting cyan (C), yellow (Y), and magenta (M) light. Whenthe subpixels 212 sensing light with four different wavelength bands areprovided in one pixel 211, the reproducibility of colors of an obtainedimage can be increased.

For example, in FIG. 36A, in regard to the subpixel 212 sensing light ina red wavelength band, the subpixel 212 sensing light in a greenwavelength band, and the subpixel 212 sensing light in a blue wavelengthband, the pixel number ratio (or the light receiving area ratio) thereofis not necessarily 1:1:1. For example, it is possible to employ theBayer arrangement, in which the ratio of the number of pixels (the ratioof light-receiving areas) is set to red:green:blue=1:2:1. Alternatively,the pixel number ratio (the ratio of light receiving area) of red andgreen to blue may be 1:6:1.

Although the number of subpixels 212 provided in the pixel 211 may beone, two or more subpixels are preferably provided. For example, whentwo or more subpixels 212 sensing light in the same wavelength band areprovided, the redundancy is increased, and the reliability of theimaging device 200 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbsor reflects visible light is used as the filter, the imaging device 200that senses infrared light can be achieved.

Furthermore, when a neutral density (ND) filter (dark filter) is used,output saturation which occurs when a large amount of light enters aphotoelectric conversion element (light-receiving element) can beprevented. With a combination of ND filters with different dimmingcapabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 211 may be provided with alens. An arrangement example of the pixel 211, a filter 254, and a lens255 is described with cross-sectional views in FIGS. 37A and 37B. Withthe lens 255, the photoelectric conversion element can receive incidentlight efficiently. Specifically, as illustrated in FIG. 37A, light 256enters a photoelectric conversion element 220 through the lens 255, thefilter 254 (a filter 254R, a filter 254G, and a filter 254B), a pixelcircuit 230, and the like which are provided in the pixel 211.

However, part of the light 256 indicated by arrows might be blocked bysome wirings 257 as indicated by a region surrounded with dashed-dottedlines. Thus, a preferable structure is such that the lens 255 and thefilter 254 are provided on the photoelectric conversion element 220 sideas illustrated in FIG. 37B, whereby the photoelectric conversion element220 can efficiently receive the light 256. When the light 256 enters thephotoelectric conversion element 220 from the photoelectric conversionelement 220 side, the imaging device 200 with high sensitivity can beprovided.

As the photoelectric conversion element 220 illustrated in FIGS. 37A and37B, a photoelectric conversion element in which a p-n junction or ap-i-n junction is formed may be used.

The photoelectric conversion element 220 may be formed using a substancethat has a function of absorbing a radiation and generating electriccharges. Examples of the substance that has a function of absorbing aradiation and generating electric charges include selenium, lead iodide,mercury iodide, gallium arsenide, cadmium telluride, and cadmium zincalloy.

For example, when selenium is used for the photoelectric conversionelement 220, the photoelectric conversion element 220 can have a lightabsorption coefficient in a wide wavelength range, such as visiblelight, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 211 included in the imaging device 200 may include thesubpixel 212 with a first filter in addition to the subpixel 212illustrated in FIGS. 37A and 37B.

<Configuration Example 2 of Pixel>

An example of a pixel including a transistor using silicon and atransistor using an oxide semiconductor according to one embodiment ofthe present invention is described below.

FIGS. 38A and 38B are each a cross-sectional view of an element includedin an imaging device.

The imaging device illustrated in FIG. 38A includes a transistor 351including silicon over a silicon substrate 300, transistors 353 and 354which include an oxide semiconductor and are stacked over the transistor351, and a photodiode 360 provided in a silicon substrate 300 andincluding an anode 361 and a cathode 362. The transistors and thephotodiode 360 are electrically connected to various plugs 370 andwirings 371. In addition, an anode 361 of the photodiode 360 iselectrically connected to the plug 370 through a low-resistance region363

The imaging device includes a layer 305 including the transistor 351provided on the silicon substrate 300 and the photodiode 360 provided inthe silicon substrate 300, a layer 320 which is in contact with thelayer 305 and includes the wirings 371, a layer 331 which is in contactwith the layer 320 and includes the transistors 353 and 354, and a layer340 which is in contact with the layer 331 and includes a wiring 372 anda wiring 373.

Note that in the example of cross-sectional view in FIG. 38A, alight-receiving surface of the photodiode 360 is provided on the sideopposite to a surface of the silicon substrate 300 where the transistor351 is formed. With the structure, an optical path can be obtainedwithout the influence by the transistors or wirings, and therefore, apixel with a high aperture ratio can be formed. Thus, a pixel with ahigh aperture ratio can be formed. Note that the light-receiving surfaceof the photodiode 360 can be the same as the surface where thetransistor 351 is formed.

In the case where a pixel is formed with use of only transistors usingan oxide semiconductor, the layer 305 may include the transistor usingan oxide semiconductor. Alternatively, the layer 305 may be omitted, andthe pixel may include only transistors using an oxide semiconductor.

In addition, in the cross-sectional view in FIG. 38A, the photodiode 360in the layer 305 and the transistor in the layer 331 can be formed so asto overlap with each other. Thus, the degree of integration of pixelscan be increased. In other words, the resolution of the imaging devicecan be increased.

An imaging device shown in FIG. 38B includes a photodiode 365 in thelayer 340 and over the transistor. In FIG. 38B, the layer 305 includesthe transistor 351 and a transistor 352 using silicon, the layer 320includes the wiring 371, the layer 331 includes the transistors 353 and354 using an oxide semiconductor layer, the layer 340 includes thephotodiode 365. The photodiode 365 includes a semiconductor layer 366, asemiconductor layer 367, and a semiconductor layer 368, and iselectrically connected to the wiring 373 and a wiring 374 through theplug 370.

The element structure shown in FIG. 38B can increase the aperture ratio.

Alternatively, a PIN diode element formed using an amorphous siliconfilm, a microcrystalline silicon film, or the like may be used as thephotodiode 365. In the photodiode 365, an n-type semiconductor layer368, an i-type semiconductor layer 367, and a p-type semiconductor layer366 are stacked in this order. The i-type semiconductor layer 367 ispreferably formed using amorphous silicon. The p-type semiconductorlayer 366 and the n-type semiconductor layer 368 can each be formedusing amorphous silicon, microcrystalline silicon, or the like whichincludes a dopant imparting the corresponding conductivity type. Thephotodiode 365 in which a photoelectric conversion layer is formed usingamorphous silicon has high sensitivity in a visible light wavelengthregion, and therefore can easily sense weak visible light.

Here, in FIGS. 38A and 38B, an insulator 380 is provided between thelayer 305 including the transistor 351 and the layer 331 including thetransistors 353 and 354. However, there is no limitation on the positionof the insulator 380.

Hydrogen in an insulator provided in the vicinity of a channel formationregion of the transistor 351 terminates dangling bonds of silicon;accordingly, the reliability of the transistor 351 can be improved. Incontrast, hydrogen in the insulator provided in the vicinity of thetransistor 353, the transistor 354, and the like becomes one of factorsgenerating a carrier in the oxide semiconductor. Thus, the hydrogen maycause a reduction of the reliability of the transistor 354, thetransistor 354, and the like. Therefore, in the case where thetransistor using an oxide semiconductor is provided over the transistorusing a silicon-based semiconductor, it is preferable that the insulator380 having a function of blocking hydrogen be provided between thetransistors. When the hydrogen is confined below the insulator 380, thereliability of the transistor 351 can be improved. In addition, thehydrogen can be prevented from being diffused from a part below theinsulator 380 to a part above the insulator 380; thus, the reliabilityof the transistor 353, the transistor 354, and the like can beincreased. It is preferable to form the insulator 381 over thetransistors 353 and 354 because oxygen diffusion can be prevented in theoxide semiconductor.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 10 Rf Tag

In this embodiment, an RF tag that includes the transistor described inthe above embodiments or the memory device described in the aboveembodiment is described with reference to FIG. 39.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have extremely high reliability in order to beused for this purpose.

A configuration of the RF tag will be described with reference to FIG.39. FIG. 39 is a block diagram illustrating a configuration example ofan RF tag.

As shown in FIG. 39, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, a structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be usedas the memory circuit 810. Since the memory circuit of one embodiment ofthe present invention can retain data even when not powered, the memorycircuit can be favorably used for an RF tag. Furthermore, the memorycircuit of one embodiment of the present invention needs power (voltage)needed for data writing significantly lower than that needed in aconventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. Furthermore, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 11 Display Device

A display device of one embodiment of the present invention is describedbelow with reference to FIGS. 40A to 40C and FIGS. 41A and 41B.

Examples of a display element provided in the display device include aliquid crystal element (also referred to as a liquid crystal displayelement) and a light-emitting element (also referred to as alight-emitting display element). The light-emitting element includes, inits category, an element whose luminance is controlled by a current orvoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like. Adisplay device including an EL element (EL display device) and a displaydevice including a liquid crystal element (liquid crystal displaydevice) are described below as examples of the display device.

Note that the display device described below includes in its category apanel in which a display element is sealed and a module in which an ICsuch as a controller is mounted on the panel.

The display device described below refers to an image display device ora light source (including a lighting device). The display deviceincludes any of the following modules: a module provided with aconnector such as an FPC or TCP; a module in which a printed wiringboard is provided at the end of TCP; and a module in which an integratedcircuit (IC) is mounted directly on a display element by a COG method.

FIGS. 40A to 40C illustrate an example of an EL display device of oneembodiment of the present invention. FIG. 40A is a circuit diagram of apixel in an EL display device. FIG. 40B is a top view showing the wholeof the EL display device.

FIG. 40C is a cross-sectional view taken along part of dashed-dottedline M-N in FIG. 40B.

FIG. 40A illustrates an example of a circuit diagram of a pixel used inan EL display device.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. Particularly in the case where the number ofportions to which a terminal is connected might be more than one, it isnot necessary to specify the portions to which the terminal isconnected. Therefore, it might be possible to constitute one embodimentof the invention by specifying only portions to which some of terminalsof an active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

The EL display device illustrated in FIG. 40A includes a switchingelement 743, a transistor 741, a capacitor 742, and a light-emittingelement 719.

Note that FIG. 40A and the like each illustrate an example of a circuitstructure; therefore, a transistor can be provided additionally. Incontrast, for each node in FIG. 40A and the like, it is possible not toprovide an additional transistor, switch, passive element, or the like.

A gate of the transistor 741 is electrically connected to one terminalof the switching element 743 and one electrode of the capacitor 742. Asource of the transistor 741 is electrically connected to the otherelectrode of the capacitor 742 and one electrode of the light-emittingelement 719. A power supply potential VDD is supplied to a drain of thetransistor 741. The other terminal of the switching element 743 iselectrically connected to a signal line 744. A constant potential issupplied to the other electrode of the light-emitting element 719. Theconstant potential is a ground potential GND or a potential lower thanthe ground potential GND.

It is preferable to use a transistor as the switching element 743. Whenthe transistor is used as the switching element, the area of a pixel canbe reduced, so that the EL display device can have high resolution. Asthe switching element 743, a transistor formed through the same step asthe transistor 741 can be used, so that EL display devices can bemanufactured with high productivity. Note that as the transistor 741and/or the switching element 743, the transistor illustrated in FIGS. 1Ato 1C can be used, for example.

FIG. 40B is a top view of the EL display device. The EL display deviceincludes a substrate 700, a substrate 750, a sealant 734, a drivercircuit 735, a driver circuit 736, a pixel 737, and an FPC 732. Thesealant 734 is provided between the substrate 700 and the substrate 750so as to surround the pixel 737, the driver circuit 735, and the drivercircuit 736. Note that the driver circuit 735 and/or the driver circuit736 may be provided outside the sealant 734.

FIG. 40C is a cross-sectional view of the EL display device taken alongpart of dashed-dotted line M-N in FIG. 40B. In FIG. 40C, the FPC 732 isconnected to the wiring 733 a via the terminal 731. Note that the wiring733 a may be formed using the same kind of conductor or semiconductor asthe conductor or semiconductor included in the transistor 741.

FIG. 40C illustrates a structure of the transistor 741 including aninsulator 712 over the transistor 700; a conductor 704 a; an insulator706 a that is over the insulator 712 a and the conductor 704 a andpartly overlaps with the conductor 704 a; a semiconductor 706 b over theinsulator 706 a; conductors 716 a 1 and 716 a 2 in contact with a topsurface of the semiconductor 706 b; an insulator 710 over the conductors716 a 1 and 716 a 2; an insulator 706 c over the semiconductor 706 b; aninsulator 718 a over the insulator 706 c; and a conductor 714 a that isover the insulator 718 a and overlaps with the semiconductor 706 b. Notethat the structure of the transistor 741 is just an example; thetransistor 741 may have a structure different from that illustrated inFIG. 40C.

In the transistor 741 illustrated in FIG. 40C, the conductor 704 aserves as a gate electrode, the insulator 712 a serves as a gateinsulator, the conductor 716 a 2 serves as a source electrode, theconductor 716 a 1 serves as a drain electrode, the insulator 718 aserves as a gate insulator, and the conductor 714 a serves as a gateelectrode. Note that in some cases, electric characteristics of theinsulator 706 a, the semiconductor 706 b, and the insulator 706 c changeif light enters the insulator 706 a, the semiconductor 706 b, and theinsulator 706 c. To prevent this, it is preferable that one or more ofthe conductor 704 a, the conductor 716 a 1, the conductor 716 a 2, andthe conductor 714 a have a light-blocking property.

FIG. 40C illustrates a structure of the capacitor 742 including aninsulator 706 d that is over a conductor 704 b and partly overlaps withthe conductor 704 b; a semiconductor 706 e over the insulator 706 d;conductors 716 a 3 and 716 a 4 in contact with a top surface of thesemiconductor 706 e; the insulator 710 over the conductors 716 a 3 and716 a 4; an insulator 706 f over the semiconductor 706 e; the insulator718 b over the insulator 706 f; and a conductor 714 b that is over theinsulator 718 b and overlaps with the semiconductor 706 e.

In the capacitor 742, the conductor 704 b serves as one electrode andthe conductor 714 b serves as the other electrode.

Thus, the capacitor 742 can be formed using a film of the transistor741. The conductor 704 a and the conductor 704 b are preferablyconductors of the same kind, in which case the conductor 704 a and theconductor 704 b can be formed through the same step. Furthermore, theconductor 714 a and the conductor 714 b are preferably conductors of thesame kind, in which case the conductor 714 a and the conductor 714 b canbe formed through the same step.

The capacitor 742 illustrated in FIG. 40C has a large capacitance perarea occupied by the capacitor. Therefore, the EL display deviceillustrated in FIG. 40C has high display quality. Note that thestructure of the capacitor 742 is just an example and may be differentfrom that illustrated in FIG. 40C.

An insulator 728 is provided over the transistor 741 and the capacitor742, and an insulator 720 is provided over the insulator 728. Here, theinsulator 728 and the insulator 720 may have an opening reaching theconductor 716 a 1 that serves as the drain electrode of the transistor741. A conductor 781 is provided over the insulator 720. The conductor781 may be electrically connected to the transistor 741 through theopening in the insulator 728 and the insulator 720.

A partition wall 784 having an opening reaching the conductor 781 isprovided over the conductor 781. A light-emitting layer 782 in contactwith the conductor 781 through the opening provided in the partitionwall 784 is provided over the partition wall 784. A conductor 783 isprovided over the light-emitting layer 782. A region where the conductor781, the light-emitting layer 782, and the conductor 783 overlap withone another serves as the light-emitting element 719.

So far, examples of the EL display device are described. Next, anexample of a liquid crystal display device is described.

FIG. 41A is a circuit diagram showing a structural example of a pixel ofthe liquid crystal display device. A pixel illustrated in FIG. 41Aincludes a transistor 751, a capacitor 752, and an element (liquidcrystal element) 753 in which a space between a pair of electrodes isfilled with a liquid crystal.

One of a source and a drain of the transistor 751 is electricallyconnected to a signal line 755, and a gate of the transistor 751 iselectrically connected to a scan line 754.

One electrode of the capacitor 752 is electrically connected to theother of the source and the drain of the transistor 751, and the otherelectrode of the capacitor 752 is electrically connected to a wiring forsupplying a common potential.

One electrode of the liquid crystal element 753 is electricallyconnected to the other of the source and the drain of the transistor751, and the other electrode of the liquid crystal element 753 iselectrically connected to a wiring to which a common potential issupplied. The common potential supplied to the wiring electricallyconnected to the other electrode of the capacitor 752 may be differentfrom that supplied to the other electrode of the liquid crystal element753.

Note the description of the liquid crystal display device is made on theassumption that the top view of the liquid crystal display device issimilar to that of the EL display device. FIG. 40B is a cross-sectionalview of the liquid crystal display device taken along dashed-dotted lineM-N in FIG. 41B. In FIG. 41B, the FPC 732 is connected to the wiring 733a via the terminal 731. Note that the wiring 733 a may be formed usingthe same kind of conductor as the conductor of the transistor 751 orusing the same kind of semiconductor as the semiconductor of thetransistor 751.

For the transistor 751, the description of the transistor 741 isreferred to. For the capacitor 752, the description of the capacitor 742is referred to. Note that the structure of the capacitor 752 in FIG. 41Bcorresponds to, but is not limited to, the structure of the capacitor742 in FIG. 40C.

Note that in the case where an oxide semiconductor is used as thesemiconductor of the transistor 751, the off-state current of thetransistor 751 can be extremely small. Therefore, an electric chargeheld in the capacitor 752 is unlikely to leak, so that the voltageapplied to the liquid crystal element 753 can be maintained for a longtime. Accordingly, the transistor 751 can be kept off during a period inwhich moving images with few motions or a still image are/is displayed,whereby power for the operation of the transistor 751 can be saved inthat period; accordingly a liquid crystal display device with low powerconsumption can be provided. Furthermore, the area occupied by thecapacitor 752 can be reduced; thus, a liquid crystal display device witha high aperture ratio or a high-resolution liquid crystal display devicecan be provided.

An insulator 721 is provided over the transistor 751 and the capacitor752. The insulator 721 has an opening reaching the transistor 751. Aconductor 791 is provided over the insulator 721. The conductor 791 iselectrically connected to the transistor 751 through the opening in theinsulator 721.

An insulator 792 serving as an alignment film is provided over theconductor 791. A liquid crystal layer 793 is provided over the insulator792. An insulator 794 serving as an alignment film is provided over theliquid crystal layer 793. A spacer 795 is provided over the insulator794. A conductor 796 is provided over the spacer 795 and the insulator794. A substrate 797 is provided over the conductor 796.

Owing to the above-described structure, a display device including acapacitor occupying a small area, a display device with high displayquality, or a high-resolution display device can be provided.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ various modes or caninclude various elements. For example, the display element, the displaydevice, the light-emitting element, or the light-emitting deviceincludes at least one of a light-emitting diode (LED) for white, red,green, blue, or the like, a transistor (a transistor that emits lightdepending on current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a display element using micro electromechanical systems (MEMS), a digital micromirror device (DMD), a digitalmicro shutter (DMS), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, a display element including a carbon nanotube, and the like.Other than the above, display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electrical ormagnetic effect may be included.

Note that examples of display devices having EL elements include an ELdisplay. Examples of a display device including an electron emitterinclude a field emission display (FED), an SED-type flat panel display(SED: surface-conduction electron-emitter display), and the like.Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of display devices having electronic ink oran electrophoretic element include electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

Note that in the case of using an LED, graphene or graphite may beprovided under an electrode or a nitride semiconductor of the LED.Graphene or graphite may be a multilayer film in which a plurality oflayers are stacked. As described above, provision of graphene orgraphite enables easy formation of a nitride semiconductor thereover,such as an n-type GaN semiconductor including crystals. Furthermore, ap-type GaN semiconductor including crystals or the like can be providedthereover, and thus the LED can be formed. Note that an AlN layer may beprovided between the n-type GaN semiconductor including crystals andgraphene or graphite. The GaN semiconductors included in the LED may beformed by MOCVD. Note that when the graphene is provided, the GaNsemiconductors included in the LED can also be formed by a sputteringmethod.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 12

In this embodiment, a display module using a semiconductor device of oneembodiment of the present invention is described with reference to FIG.42.

<Display Module>

In a display module 6000 in FIG. 42, a touch panel 6004 connected to anFPC 6003, a display panel 6006 connected to an FPC 6005, a backlightunit 6007, a frame 6009, a printed board 6010, and a battery 6011 areprovided between an upper cover 6001 and a lower cover 6002. Note thatthe backlight unit 6007, the battery 6011, the touch panel 6004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 6006 and an integratedcircuit mounted on a printed circuit board.

The shapes and sizes of the upper cover 6001 and the lower cover 6002can be changed as appropriate in accordance with the sizes of the touchpanel 6004 and the display panel 6006.

The touch panel 6004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 6006. Acounter substrate (sealing substrate) of the display panel 6006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 6006 so that an optical touch panel function is added.An electrode for a touch sensor may be provided in each pixel of thedisplay panel 6006 so that a capacitive touch panel function is added.

The backlight unit 6007 includes a light source 6008. The light source6008 may be provided at an end portion of the backlight unit 6007 and alight diffusing plate may be used.

The frame 6009 protects the display panel 6006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated fromthe printed board 6010. The frame 6009 may function as a radiator plate.

The printed board 6010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 6011 provided separatelymay be used. Note that the battery 6011 is not necessary in the casewhere a commercial power source is used.

The display module 6000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 13 Package Using a Lead Frame Interposer

FIG. 43A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer. In the package illustratedin FIG. 43A, a chip 551 corresponding to the semiconductor device of oneembodiment of the present invention is connected to a terminal 552 overan interposer 550 by wire bonding. The terminal 552 is placed on asurface of the interposer 550 on which the chip 551 is mounted. The chip551 may be sealed by a mold resin 553, in which case the chip 551 issealed such that part of each of the terminals 552 is exposed.

FIG. 43B illustrates the structure of a module of an electronic device(mobile phone) in which a package is mounted on a circuit board. In themodule of the mobile phone in FIG. 43B, a package 602 and a battery 604are mounted on a printed wiring board 601. The printed wiring board 601is mounted on a panel 600 including a display element by an FPC 603.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 14

In this embodiment, electronic devices and lighting devices of oneembodiment of the present invention will be described with reference todrawings.

<Electronic Device>

Electronic devices and lighting devices can be fabricated using thesemiconductor device of one embodiment of the present invention. Inaddition, highly reliable electronic devices and lighting devices can befabricated using the semiconductor device of one embodiment of thepresent invention. Furthermore, electronic devices and lighting devicesincluding touch sensors with improved detection sensitivity can befabricated using the semiconductor device of one embodiment of thepresent invention.

Examples of electronic devices include a television set (also referredto as a television or a television receiver), a monitor of a computer orthe like, a digital camera, a digital video camera, a digital photoframe, a mobile phone (also referred to as a mobile phone device), aportable game machine, a portable information terminal, an audioreproducing device, a large game machine such as a pinball machine, andthe like.

In the case of having flexibility, the electronic device or lightingdevice of one embodiment of the present invention can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

As examples of the secondary battery, a lithium ion secondary batterysuch as a lithium polymer battery (lithium ion polymer battery) using agel electrolyte, a lithium ion battery, a nickel-hydride battery, anickel-cadmium battery, an organic radical battery, a lead-acid battery,an air secondary battery, a nickel-zinc battery, and a silver-zincbattery can be given.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for contactless power transmission.

FIG. 44A illustrates a portable game machine including a housing 7101, ahousing 7102, a display portion 7103, a display portion 7104, amicrophone 7105, speakers 7106, an operation key 7107, a stylus 7108,and the like. The semiconductor device of one embodiment of the presentinvention can be used for an integrated circuit, a CPU, or the likeincorporated in the housing 7101. When the light-emitting device of oneembodiment of the present invention is used as the display portion 7103or 7104, it is possible to provide a user-friendly portable game machinewith quality that hardly deteriorates. Although the portable gamemachine illustrated in FIG. 44A includes two display portions, thedisplay portion 7103 and the display portion 7104, the number of displayportions included in the portable game machine is not limited to two.

FIG. 44B illustrates a smart watch, which includes a housing 7302, adisplay portion 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like. Thesemiconductor device of one embodiment of the present invention can beused for a memory, a CPU, or the like incorporated in the housing 7302.

FIG. 44C illustrates a portable information terminal, which includes adisplay portion 7502 incorporated in a housing 7501, operation buttons7503, an external connection port 7504, a speaker 7505, a microphone7506, a display portion 7502, and the like. The semiconductor device ofone embodiment of the present invention can be used for a mobile memory,a CPU, or the like incorporated in the housing 7501. Note that thedisplay portion 7502 is small- or medium-sized but can perform full highvision, 4 k, or 8 k display because it has greatly high definition;therefore, a significantly clear image can be obtained.

FIG. 44D illustrates a video camera, which includes a first housing7701, a second housing 7702, a display portion 7703, operation keys7704, a lens 7705, a joint 7706, and the like. The operation keys 7704and the lens 7705 are provided for the first housing 7701, and thedisplay portion 7703 is provided for the second housing 7702. The firsthousing 7701 and the second housing 7702 are connected to each otherwith the joint 7706, and the angle between the first housing 7701 andthe second housing 7702 can be changed with the joint 7706. Imagesdisplayed on the display portion 7703 may be switched in accordance withthe angle at the joint 7706 between the first housing 7701 and thesecond housing 7702. The imaging device in one embodiment of the presentinvention can be provided in a focus position of the lens 7705. Thesemiconductor device of one embodiment of the present invention can beused for an integrated circuit, a CPU, or the like incorporated in thefirst housing 7701.

FIG. 44E illustrates a digital signage including a display portion 7902provided on a utility pole 7901. The display device of one embodiment ofthe present invention can be used for a control circuit of the displayportion 7902.

FIG. 45A illustrates a notebook personal computer, which includes ahousing 8121, a display portion 8122, a keyboard 8123, a pointing device8124, and the like. The semiconductor device of one embodiment of thepresent invention can be used for a CPU, a memory, or the likeincorporated in the housing 8121. Note that the display portion 8122 issmall- or medium-sized but can perform 8 k display because it hasgreatly high definition; therefore, a significantly clear image can beobtained.

FIG. 45B is an external view of an automobile 9700. FIG. 45C illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The semiconductor device of one embodiment of the present invention canbe used in a display portion and a control integrated circuit of theautomobile 9700. For example, the semiconductor device of one embodimentof the present invention can be used in display portions 9710 to 9715illustrated in FIG. 45C.

The display portion 9710 and the display portion 9711 are displaydevices or input/output devices provided in an automobile windshield.The display device or input/output device of one embodiment of thepresent invention can be a see-through display device or input/outputdevice, through which the opposite side can be seen, by using alight-transmitting conductive material for its electrodes. Such asee-through display device or input/output device does not hinderdriver's vision during the driving of the automobile 9700. Therefore,the display device or input/output device of one embodiment of thepresent invention can be provided in the windshield of the automobile9700. Note that in the case where a transistor or the like for drivingthe display device or input/output device is provided in the displaydevice or input/output device, a transistor having light-transmittingproperties, such as an organic transistor using an organic semiconductormaterial or a transistor using an oxide semiconductor, is preferablyused.

The display portion 9712 is a display device provided on a pillarportion. For example, an image taken by an imaging unit provided in thecar body is displayed on the display portion 9712, whereby the viewhindered by the pillar portion can be compensated. The display portion9713 is a display device provided on the dashboard.

For example, an image taken by an imaging unit provided in the car bodyis displayed on the display portion 9713, whereby the view hindered bythe dashboard can be compensated. That is, by displaying an image takenby an imaging unit provided on the outside of the automobile, blindareas can be eliminated and safety can be increased. Displaying an imageto compensate for the area which a driver cannot see, makes it possiblefor the driver to confirm safety easily and comfortably.

FIG. 45D illustrates the inside of a car in which a bench seat is usedas a driver seat and a front passenger seat. A display portion 9721 is adisplay device or input/output device provided in a door portion. Forexample, an image taken by an imaging unit provided in the car body isdisplayed on the display portion 9721, whereby the view hindered by thedoor can be compensated. A display portion 9722 is a display deviceprovided in a steering wheel. A display portion 9723 is a display deviceprovided in the middle of a seating face of the bench seat. Note thatthe display device can be used as a seat heater by providing the displaydevice on the seating face or backrest and by using heat generation ofthe display device as a heat source.

The display portion 9714, the display portion 9715, and the displayportion 9722 can display a variety of kinds of information such asnavigation data, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be changed freely bya user as appropriate. The information listed above can also bedisplayed on the display portions 9710 to 9713, 9721, and 9723. Thedisplay portions 9710 to 9715 and 9721 to 9723 can also be used aslighting devices. The display portions 9710 to 9715 and 9721 to 9723 canalso be used as heating devices.

FIG. 46A illustrates an external view of a camera 8000. The camera 8000includes a housing 8001, a display portion 8002, an operation button8003, a shutter button 8004, a connection portion 8005, and the like. Alens 8006 can be put on the camera 8000.

The connection portion 8005 includes an electrode to connect with afinder 8100, which is illustrated in FIG. 46B, a stroboscope, or thelike.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in ahousing.

Images can be taken at the touch of the shutter button 8004. Inaddition, images can be taken at the touch of the display portion 8002which serves as a touch panel.

The display device or input/output device of one embodiment of thepresent invention can be used in the display portion 8002.

FIG. 46B shows the camera 8000 with the finder 8100 connected.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a connection portion for the camera 8000 andthe connection portion 8005, and the finder 8100 can be connected to thecamera 8000. The connection portion includes an electrode, and an imageor the like received from the camera 8000 through the electrode can bedisplayed on the display portion 8102.

The button 8103 has a function of a power button, and the displayportion 8102 can be turned on and off with the button 8103.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit and an image sensor included in thehousing 8101.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIGS. 46A and 46B, the housing 8001 of the camera8000 may include a finder having the display device or input/outputdevice of one embodiment of the present invention.

FIG. 46C illustrates an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. In addition, the movement of the eyeball and theeyelid of a user can be captured by a camera in the main body 8203 andthen coordinates of the points the user looks at can be calculated usingthe captured data to utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes to be incontact with the user. The main body 8203 may be configured to sensecurrent flowing through the electrodes with the movement of the user'seyeball to recognize the direction of his or her eyes. The main body8203 may be configured to sense current flowing through the electrodesto monitor the user's pulse. The mounting portion 8201 may includesensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit included in the main body 8203.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 15

In this embodiment, application examples of an RF tag using thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIGS. 47A to 47F.

<Application Examples of RF Tag>

The RF tag is widely used and can be provided for, for example, productssuch as bills, coins, securities, bearer bonds, documents (e.g.,driver's licenses or resident's cards, see FIG. 47A), vehicles (e.g.,bicycles, see FIG. 47B), packaging containers (e.g., wrapping paper orbottles, see FIG. 47C), recording media (e.g., DVD or video tapes),personal belongings (e.g., bags or glasses, see FIG. 47D), foods,plants, animals, human bodies, clothing, household goods, medicalsupplies such as medicine and chemicals, and electronic devices (e.g.,liquid crystal display devices, EL display devices, television sets, orcellular phones), or tags on products (see FIGS. 47E and 47F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 of one embodiment of the present invention can be reduced insize, thickness, and weight, it can be fixed to a product withoutspoiling the design of the product. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have anidentification function by being provided with the RF tag 4000 of oneembodiment of the present invention, and the identification function canbe utilized to prevent counterfeiting. Moreover, the efficiency of asystem such as an inspection system can be improved by providing the RFtag of one embodiment of the present invention for packaging containers,recording media, personal belongings, foods, clothing, household goods,electronic devices, or the like. Vehicles can also have higher securityagainst theft or the like by being provided with the RF tag of oneembodiment of the present invention.

As described above, by using the RF tag including the semiconductordevice of one embodiment of the present invention for each applicationdescribed in this embodiment, power for operation such as writing orreading of data can be reduced, which results in an increase in themaximum communication distance. Moreover, data can be held for anextremely long period even in the state where power is not supplied;thus, the RF tag can be preferably used for application in which data isnot frequently written or read.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

This application is based on Japanese Patent Application serial no.2015-044526 filed with Japan Patent Office on Mar. 6, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a first insulator over a substrate; a second insulator over the first insulator; a semiconductor over the second insulator, the semiconductor including a first region, a second region, and a third region, wherein the first region is located between the second region and the third region; a source electrode and a drain electrode over the second region and the third region of the semiconductor, respectively; a third insulator over and in contact with the first region of the semiconductor; a fourth insulator over the source electrode and the drain electrode; a first gate insulator including a first region over a top surface of the third insulator and a second region over and in contact with a side surface of the fourth insulator; and a first gate electrode over the first gate insulator, wherein an entirety of the third insulator is directly underneath the first gate electrode, wherein the first insulator has a first region in contact with the second insulator and a second region in contact with one of the source electrode and the drain electrode, and wherein a thickness of the first region of the first insulator is larger than a thickness of the second region of the first insulator.
 2. The semiconductor device according to claim 1, wherein the third insulator has a stacked-layer structure including at least a first layer and a second layer.
 3. The semiconductor device according to claim 1, wherein the semiconductor includes a first oxide semiconductor.
 4. The semiconductor device according to claim 1, wherein the semiconductor includes a first oxide semiconductor having a c-axis aligned crystal part.
 5. The semiconductor device according to claim 1, wherein each of the source electrode and the drain electrode has a stacked-layer structure including a plurality of layers.
 6. The semiconductor device according to claim 1, wherein each of the source electrode and the drain electrode includes regions with different thicknesses.
 7. The semiconductor device according to claim 1, further comprising a second gate electrode under the first insulator.
 8. The semiconductor device according to claim 1, wherein each of the second insulator and the third insulator contains at least one element contained in the semiconductor.
 9. The semiconductor device according to claim 1, wherein the semiconductor has electron affinity higher than the second insulator and the third insulator. 